To return, however, to Lamarck.
"Though Nature," he continues, "in the course of long time has evolved all animals and plants in a true scale of progression, the steps of this scale can be perceived only in the princ.i.p.al groups of living forms; it cannot be perceived in species nor even in genera. The reason of this lies in the extreme diversity of the surroundings in which each different race of animals and plants has existed. These surroundings have often been out of harmony with the growing organization of the plants and animals themselves; this has led to anomalies, and, as it were, digressions, which the mere development of organization by itself could not have occasioned."[256] Or, in other words, to that divergency of type which is so well insisted on by Mr. Charles Darwin.
"It is only therefore the princ.i.p.al groups of animal and vegetable life which can be arranged in a vertical line of descent; species and even genera cannot always be so--for these contain beings whose organization has been dependent on the possession of such and such a special system of essential organs.
"Each great and separate group has its own system of essential organs, and it is these systems which can be seen to descend, within the limits of the group, from their most complex to their simplest form. But each organ, considered individually, does not descend by equally regular gradation; the gradations are less and less regular according as the organ is of less importance, and is more susceptible of modification by the conditions which surround it. Organs of small importance, and not essential to existence, are not always either perfected or degraded at an equal rate, so that in observing all the species of any cla.s.s we find an organ in one species in the highest degree of perfection, while another organ, which in this same species is impoverished or very imperfect, is highly developed in another species of the same group."[257]
The facts maintained in the preceding paragraph are in great measure supported by Mr. Charles Darwin, who, however, a.s.signs their cause to natural selection.
Mr. Darwin writes, "Ordinary specific characters are more variable than generic;" and again, a little lower down, "The points in which all the species of a genus resemble each other, and in which they differ from allied genera, are called generic characters; and these characters may be attributed to inheritance from a common progenitor, for it can rarely happen that natural selection will have modified several distinct species fitted to more or less widely different habits, in exactly the same manner; and as these so called generic characters have been inherited from before the period when the several species first branched off from their common progenitor, and subsequently have not varied or come to differ in any degree, or only in a slight degree, it is not probable that they should vary at the present day. On the other hand, the points in which species differ from other species of the same genus are called specific characters; and as these specific characters have varied and come to differ since the period when the species branched off from a common progenitor, it is probable that they should still often be in some degree variable, or at least more variable than those parts of the organization which have for a very long time remained constant."[258]
The fact, then, that it is specific characters which vary most is agreed upon by both Lamarck and Mr. Darwin. Lamarck, however, maintains that it is these specific characters which are most capable of being affected by the habits of the creature, and that it is for this reason they will be most variable, while Mr. Darwin simply says they _are_ most variable, and that, this being so, the favourable variations will be preserved and acc.u.mulated--an a.s.sertion which Lamarck would certainly not demur to.
"Irregular degrees of perfection," says Lamarck, "and degradation in the less essential organs, are due to the fact that these are more liable than the more essential ones to the influence of external circ.u.mstances: these induce corresponding differences in the more outward parts of the animal, and give rise to such considerable and singular difference in species, that instead of being able to arrange them in a direct line of descent, as we can arrange the main groups, these species often form lateral ramifications round about the main groups to which they belong, and in their extreme development are truly isolated."[259]
In his summary of the second chapter of his "Origin of Species," Mr.
Darwin well confirms this when he says, "In large genera the species are apt to be closely, but unequally, allied together, forming little cl.u.s.ters round other species."
"A longer time," says Lamarck, "and a greater influence of surrounding conditions, is necessary in order to modify interior organs.
Nevertheless we see that Nature does pa.s.s from one system to another without any sudden leap, when circ.u.mstances require it, provided the systems are not too far apart. Her method is to proceed from the more simple to the more complex.[260]
"She does this not only in the race, but in the individual." Here Lamarck, like Dr. Erasmus Darwin, shows his perception of the importance of embryology in throwing light on the affinities of animals--as since more fully insisted on by the author of the "Vestiges of Creation," and by Mr. Darwin,[261] as well as by other writers. "Breathing through gills is nearer to breathing through lungs than breathing through trachea is. Not only do we see Nature pa.s.s from gills to lungs in families which are not too far apart, as may be seen by considering the case of fishes and reptiles; but she does so during the existence of a single individual, which may successively make use both of the one and of the other system. The frog while yet a tadpole breathes through gills; on becoming a frog it breathes through lungs; but we cannot find that Nature in any case pa.s.ses from trachea to lungs."[262]
Lamarck now rapidly reviews previous cla.s.sifications, and propounds his own, which stands thus:--I. Vertebrata, consisting of Mammals, Birds, Fishes, and Reptiles. II. Invertebrata, consisting of Molluscs, Centipedes, Annelids, Crustacea, Arachnids, Insects, Worms, Radiata, Polyps, Infusoria.
"The degradation of organism," he concludes, "in this descending scale is not perfectly even, and cannot be made so by any cla.s.sification, nevertheless there is such evidence of sustained degradation in the princ.i.p.al groups as must point in the direction of some underlying general principle."[263]
Lamarck"s sixth chapter is headed "Degradation and Simplification of the Animal Chain as we proceed downwards from the most complex to the most simple Organisms."
"This is a positive fact, and results from the operation of a constant law of nature; but a disturbing cause, which can be easily recognized, varies the regular operation of the law from one end to the other of the chain of life.[264]
"We can see, nevertheless, that special organs become more and more simple the lower we descend; that they become changed, impoverished, and attenuated little by little; that they lose their local centres, and finally become definitely annihilated before we reach the lowest extremity of the chain.[265]
"As has been said already, the degradation of organism is not always regular; such and such an organ often fails or changes suddenly, and sometimes in its changes a.s.sumes forms which are not allied with any others by steps that we can recognize. An organ may disappear and reappear several times before being entirely lost: but this is what we might expect, for the cause which has led to the evolution of living organisms has evolved many varieties, due to external influences.
Nevertheless, looking at organization broadly, we observe a descending scale."[266]
"If the tendency to progressive development was the only cause which had influenced the forms and organs of animals, development would have been regular throughout the animal chain; but it has not been so: Nature is compelled to submit her productions to an environment which acts upon them, and variation in environment will induce variation in organism: this is the true cause of the sometimes strange deviations from the direct line of progression which we shall have to observe.[267]
"If Nature had only called aquatic beings into existence, and if these beings had lived always in the same climate, in the same kind of water, and at the same depth, the organization of these animals would doubtless have presented an even and regular scale of development. But there has been fresh water, salt water, running and stagnant water, warm and cold climates, an infinite variety of depth: animals exposed to these and other differences in their surroundings have varied in accordance with them.[268] In like manner those animals which have been gradually fitted for living in air instead of water have been subjected to an endless diversity in their surroundings. The following law, then, may be now propounded, namely:--
"_That anomalies in the development of organism are due to the influences of the environment and to the habits of the creature._[269]
"Some have said that the anomalies above mentioned are so great as to disprove the existence of any scale which should indicate descent; but the nearer we approach species, the smaller we see differences become, till with species itself we find them at times almost imperceptible."[270]
Lamarck here devotes about seventy pages to a survey of the animal kingdom in its entirety, beginning with the mammals and ending with the infusoria. He points out the manner in which organ after organ disappears as we descend the scale, till we are left with a form which, though presenting all the characteristics of life, has yet no special organ whatever. I am obliged to pa.s.s this cla.s.sification over, but do so very unwillingly, for it is ill.u.s.trative of Lamarck, both at his best and at his worst.
The seventh chapter is headed--
"On the influence of their surroundings on the actions and habits of animals, and on the effect of these habits and actions in modifying their organization."
"The effect of different conditions of our organization upon our character, tendencies, actions, and even our ideas, has been often remarked, but no attention has yet been paid to that of our actions and habits upon our organization itself. These actions and habits depend entirely upon our relations to the surroundings in which we habitually exist; we shall have occasion, therefore, to see how great is the effect of environment upon organization.
"But for our having domesticated plants and animals we should never have arrived at the perception of this truth; for though the influence of the environment is at all times and everywhere active upon all living bodies, its effects are so gradual that they can only be perceived over long periods of time.[271]
"Taking the chain of life in the inverse order of nature--that is to say, from man downwards--we certainly perceive a sustained but irregular degradation of organism, with an increasing simplicity both in organism and faculties.
"This fact should throw light upon the order taken by nature, but it does not show us why the gradation is so irregular, nor why throughout its extent we find so many anomalies or digressions which have apparently no order at all in their manifold varieties.[272] The explanation of this must be sought for in the infinite diversity of circ.u.mstances under which organisms have been developed. On the one hand, there is a tendency to a regular progressive development; on the other, there is a host of widely different surroundings which tend continually to destroy the regularity of development.
"It is necessary to explain what is meant by such expressions as "the effect of its environment upon the form and organization of an animal."
It must not be supposed that its surroundings directly effect any modification whatever in the form and organization of an animal.[273]
Great changes in surroundings involve great changes in the wants of animals, and these changes in their wants involve corresponding changes in their actions. If these new wants become permanent, or of very long duration, the animals contract new habits, which last as long as the wants which gave rise to them.[274] A great change in surroundings, if it persist for a long time, must plainly, therefore, involve the contraction of new habits. These new habits in their turn involve a preference for the employment of such and such an organ over such and such another organ, and in certain cases the total disuse of an organ which is no longer wanted. This is perfectly self-evident.[275]
"On the one hand, new wants have rendered a part necessary, which part has accordingly been created by a succession of efforts: use has kept it in existence, gradually strengthening and developing it till in the end it attains a considerable degree of perfection. On the other, new circ.u.mstances having in some cases rendered such or such a part useless, disuse has led to its gradually ceasing to receive the development which the other parts attain to; on this it becomes reduced, and in time disappears.[276]
"Plants have neither actions nor habits properly so called, nevertheless they change in a changed environment as much as animals do. This is due to changes in nutrition, absorption and transpiration, to degrees of heat, light, and moisture, and to the preponderance over others which certain of the vital functions attain to."
Lamarck is led into the statement that plants have neither actions nor habits, by his theories about the nervous system and the brain. Plain matter-of-fact people will prefer the view taken by Buffon, Dr. Darwin, and, more recently, by Mr. Francis Darwin, that there is no radical difference between plants and animals.
"The differences between well-nourished and ill-nourished plants become little by little very noticeable. If individuals, whether animal or vegetable, are continually ill-fed and exposed to hardships for several generations, their organization becomes eventually modified, and the modification is transmitted until a race is formed which is quite distinct from those descendants of the common parent stock which have been placed in favourable circ.u.mstances.[277] In a dry spring the meagre and stunted herbage seeds early. When, on the other hand, the spring is warm but with occasional days of rain, there is an excellent hay-crop.
If, however, any cause perpetuates unfavourable circ.u.mstances, plants will vary correspondingly, first in appearance and general conditions, and then in several particulars of their actual character, certain organs having received more development than others, these differences will in the course of time become hereditary.[278]
"Nature changes a plant or animal"s surroundings gradually--man sometimes does so suddenly. All botanists know that plants vary so greatly under domestication that in time they become hardly recognizable. They undergo so much change that botanists do not at all like describing domesticated varieties. Wheat itself is an example.
Where can wheat be found as a wild plant, unless it have escaped from some neighbouring cultivation? Where are our cauliflowers, our lettuces, to be found wild, with the same characters as they possess in our kitchen gardens?
"The same applies to our domesticated breeds of animals. What a variety of breeds has not man produced among fowls and pigeons, of which we can find no undomesticated examples!"[279]
The foregoing remarks on the effects of domestication seem to have been inspired by those given p. 123 and pp. 168, 169 of this volume.[280]
"Some, doubtless, have changed less than others, owing to their having undergone a less protracted domestication, and a less degree of change in climate; nevertheless, though our ducks and geese, for example, are of the same type as their wild progenitors, they have lost the power of long and sustained flight, and have become in other respects considerably modified.[281]
"A bird, after having been kept five or six years in a cage, cannot on being liberated fly like its brethren which have been always free. Such a change in a single lifetime has not effected any transmissible modification of type; but captivity, continued during many successive generations, would undoubtedly do so. If to the effects of captivity there be added also those of changed climate, changed food, and changed actions for the purpose of laying hold of food, these, united together and become constant, would in the course of time develop an entirely new breed."
This, again, is almost identical with the pa.s.sage from Buffon,[282] p.
148 of this volume. See also pp. 169, 170.
"Where can our many domestic breeds of dogs be found in a wild state?
Where are our bulldogs, greyhounds, spaniels, and lapdogs, breeds presenting differences which, in wild animals, would be certainly called specific? These are all descended from an animal nearly allied to the wolf, if not from the wolf itself. Such an animal was domesticated by early man, taken at successive intervals into widely different climates, trained to different habits, carried by man in his migrations as a precious capital into the most distant countries, and crossed from time to time with other breeds which had been developed in similar ways.
Hence our present multiform breeds."[283]
Here, also, it is impossible to forget Buffon"s pa.s.sages on the dog, given pp. 121, 122. See also p. 223.
"Observe the gradations which are found between the _ranunculus aquatilis_ and the _ranunculus hederaceus_: the latter--a land plant--resembles those parts of the former which grow above the surface of the water, but not those that grow beneath it.[284]
"The modifications of animals arise more slowly than those of plants; they are therefore less easily watched, and less easily a.s.signable to their true causes, but they arise none the less surely. As regards these causes, the most potent is diversity of the surroundings in which they exist, but there are also many others.[285]
"The climate of the same place changes, and the place itself changes with changed climate and exposure, but so slowly that we imagine all lands to be stable in their conditions. This, however, is not true; climatic and other changes induce corresponding changes in environment and habit, and these modify the structure of the living forms which are subjected to them. Indeed, we see intermediate forms and species corresponding to intermediate conditions.
"To the above causes must be ascribed the infinite variety of existing forms, independently of any tendency towards progressive development."[286]