EMBRYOLOGY. _Page_ 131.
I have not felt warranted to include in the text any summary of results secured by the important, but very difficult, investigations concerning the growth of animal life in the womb. This whole department of inquiry is in such an unfinished and uncertain state, that there is not warrant to found upon the evidence already obtained any general argument as to its bearing on a theory of evolution. The most competent observers admit that they are perplexed by facts ascertained, and confess that they can not as yet offer an explanation. To others all is as plain as possible; embryology supplies a convincing proof of the accuracy of an evolution theory; but these are scientific theorists who see by imagination, and are impatient of uncertainty. There are certain general considerations which must interpose difficulties in the way of constructing an argument from Embryology to evolution of species. (1) The action of environment before birth is altogether different from the action of environment after birth. (2) The theory of the evolution of species emphasizes this difference by insisting on the struggle for existence. (3) This difference being admitted, an argument from the one to the other can not hold. In the line of discovery the point of chief interest has been the fact that in some cases embryonic life shows a transition through lower forms a.n.a.logous to lower orders of animal existence prior to reaching the mature stage when birth occurs. But in connection with the facts ascertained, two things are to be remarked.
(1) Evidence of transition is most striking in the history of animal life developed external to the parental life, as in the transition from _larvae_ to _pupae_ among insects, and in the changes in the life of the tadpole. (2) If it be admitted that there is a common plan of structure for all organism, it is implied that there must be similarities in process of development. The question requiring answer, therefore, is whether in the gradual development from the germ, any further resemblance to lower orders appears than is to be antic.i.p.ated on the admission of a common plan for organic structure. There are singular examples of transition. But there are no ill.u.s.trations of uniform progress in the case of the higher orders such as would warrant the supposition that a history of evolution of the species can be read in the development of the ftus. The supposition has, however, found currency in not a little of our scientific teaching. The incompleteness of this evidence may appear from examples. Take the tadpole. Huxley states the facts thus,--"The tadpole is first a fish, then a tailed amphibian, provided with gills and lungs, before it became a frog." This is development outside parental life, and does not belong to evidence in Embryology. Confining attention to embryonic life, let us take Huxley"s statement, biologists "trace back the dog"s and the man"s development, and they find that at a certain stage of their existence, the two creatures are not distinguishable the one from the other." What is the inference to be drawn? If the two are not distinguishable, our powers of distinguishing are insufficient, for no biologist suggests that the two are alike. The difficulty of distinguishing two germs, or two examples of ftus, is a.n.a.logous to the difficulty which Darwin has pointed out of distinguishing the orders of dogs when they are six-days-old puppies, or the breed of three-days-old colt, or of nestling pigeons. At these stages, the animals may be so similar, that it is hardly possible to distinguish them, and yet in the full grown state they are quite different (Darwin"s _Origin of Species_, sixth edition, p. 391). Mr.
Darwin has presented the outstanding facts thus;--"The very general, though not universal, difference in structure between the embryo and the adult;--the various parts in the same individual embryo, which ultimately become very unlike and serve for diverse purposes, being at an early period of growth alike;--the common, but not invariable resemblance between the embryos or larvae of the most distinct species in the same cla.s.s;--the embryo often retaining whilst within the egg or womb, structures which are of no service to it, either at that or at a later period of life."
VI.
NON-ADVANCEMENT OF LOWER ORDERS. _Page_ 158.
Mr. Darwin"s answer to the difficulty put by Aga.s.siz is this;--"On our theory the continued existence of lowly organisms offers no difficulty; for natural selection, or the survival of the fittest, does not necessarily include progressive development,--it only takes advantage of such variations as arise and are beneficial to each creature under its complex relations of life."--_Origin of Species_, sixth edition, p. 98.
This wears the aspect of a limitation of the theory, and to that extent an acknowledgment of the force of the reasoning of Aga.s.siz.
VII.
PROTOPLASM. _Page_ 131.
"Protoplasm, simple or nucleated, is the formal basis of all life;" thus "all living forms are fundamentally of one character." "All the forms of Protoplasm which have yet been examined contain the four elements, carbon, hydrogen, oxygen, and nitrogen, in very complex union." Thus there is "a general uniformity in the character of the Protoplasm, or physical basis of life, in whatever group of living beings it may be studied."--HUXLEY"S _Lay Sermons_, p. 142.
VIII.
NUMBER OF SPECIES OF INSECTS. _Page_ 193.
Professor Huxley mentions that "Gerstsaecker in the new edition of Broun"s "Thier-Reich" gives 200,000 as the total number of species of _Arthropoda_." In this connection Mr. McLauchlan, when claiming that there are 200,000 species of Insects, adds, "In one order alone (_Coleoptera_) it is estimated that 80,000 species have been described."--_Nature_, xv. p. 275.
IX.
FERTILIZATION OF FLOWERS BY INSECTS. _Page_ 170.
Dr. Hermann Muller"s Observations are described in _Nature_, vol. xiv.
p. 175; vol. xv. pp. 317, 473; vol. xvi. pp. 265, 507.
X.
ANTS. _Page 192._
Mr. McCook"s Observations are summarized in _Nature_, vol. xvii. p.
433.
XI.
LIKENESS OF THE APE"S BRAIN TO THE HUMAN BRAIN. _Page_ 225.
The close resemblance of the brain of the ape to that of man, has been held to prove that the ape comes next to man in intelligence. But the facts bearing on this suggestion are fitted to occasion serious perplexity to its upholders. First stands the resemblance of bodily structure as largely explaining similarity of brain. The results of electric stimulation of the monkey"s brain lend additional force to this consideration. Again, facts are wanting to support the claim for superior intelligence in behalf of the monkey and ape. The habits of the ape in its natural state afford little evidence of an encouraging kind.
The ape gathers together a few sticks for a nest, in comparison with which the work of very small birds presents marvels of architecture. And nest-building seems the highest evidence gathered from the natural habits of the animal, when we compare it with leaning the back against a tree for rest, or staunching the blood of a wound. In the captive state the ape gives no such evidence of superior intelligence as the similarity of its brain to the human, would lead us to expect, if brain structure afford the test of intellectual power. Even after allowance has been made for sudden transition from the wild state to the captive, the evidence of capability does not appear which the theory requires.
The highest results reached by training monkeys, do not support a claim for intellectual superiority. These are mainly forms of mimicry, generally inferior to the efforts of some other animals. Add to these considerations the evidence as to the singular intelligence shown by ants, and the theory which measures intellect by brain structure is placed at a great disadvantage. Whether science may not ere long point to some theory of mind connected with animal existence must be matter of uncertainty. If, however, the easy and familiar operations of our own intelligence are a.n.a.lyzed and cla.s.sified; and if a statement of the ascertained functions of the brain is laid alongside, it will appear that nothing known to us in the action of brain, can supply a science of the operations of the human mind.
XII.
THE LARGE SIZED OR MULTIPOLAR CELLS. _Page_ 257.
On the functions of the large sized cells, it seems desirable to add a few words as to the direction in which evidence as to their functions actually points. For this purpose, a further quotation is desirable, referring to the number of fibres or processes pa.s.sing off from these large cells, distinguishing those which branch out into a fine net-work, and those which pa.s.s directly to a nerve fibre. "One at least of the processes of a multipolar nerve cell does not branch, but becomes directly continuous with a nerve fibre, and has been named the axial-cylinder process."--Professor Turner"s _Human Anatomy_, i. 201.
This taken with the facts given in Lecture VII, seems to favor the conclusions, (1) that the large cell spreads nerve energy through the tissue of the brain, while each has at least one direct line of communication with the system of nerve fibres; (2) that the large cell has intimate and extended relations with the motor system.
XIII.
THE CONCEPTION OF DUTY. _Page_ 273.
"Duty! Thou great, thou exalted name! Wondrous thought that workest neither by fond insinuation, flattery, nor by any threat, but by merely holding up thy naked law in the soul, and so extorting for thyself always reverence, if not always obedience,--before whom all appet.i.tes are dumb, however secretly they rebel,--whence thy original?"--KANT"S _Critique of Practical Reason_.