[18] Mr. Wallace has recently, in his work on “Darwinism,” taken strong ground against this Darwinian factor. He thinks, for example, that sexual vigor is the cause of both the splendor of color and the pertinacity which secures the female. We see little difference in this way of putting it. Our object, however, is not to argue the question of what are true factors, but simply to give the most accepted, and, as it seems to us, also the most probable view.
[19] By reason I mean the faculty of dealing with the phenomena of the inner world of consciousness and ideas. Animals live in one world—the outer world of sense; man in two—the outer world of sense, like animals, but also in an inner and higher world of ideas. All that is characteristic of man comes of this capacity of dealing with the inner world. In default of a better word I call it reason. If any one can suggest a better word, I will gladly adopt it.
[20] While all comparative anatomists agree that the lung is a diverticulum from the œsophagus, like the air-bladder of the gar-fish, some think that it is a different diverticulum, which is seen first in the dipnoi.
[21] Undoubtedly the true principle on which primary groups ought to be made is, identity of general plan of structure, or traceableness of homology throughout. For these groups are the great primary branches of the tree of life, and classification ought to represent degrees of genetic relationship. This was Agassiz’s principle, although he did not admit the genetic relation. This principle has been, it seems to us, too much neglected by later systematists.
[22] The Amphioxus, the lowest of all vertebrates—if vertebrate it may be called—is an exception to 2 and 3. In this animal the vertebrate type is not yet fully declared.
[23] This is only one example under a general law which it may be well to stop a moment to illustrate. A repetition of similar parts performing the same function is always an evidence of low organization, and as we rise in the scale of organization such parts usually become fewer and more efficient. Thus, to give one example, myriapods, as their name indicates, have hundreds of locomotive organs—lower crustaceans perhaps thirty or forty. As we go up, they are reduced to fourteen (tetradecapods), then to ten (decapods), then in spiders to eight, in insects to six, in vertebrates to four, and in man to two. A similar reduction in number, but increase in efficiency, is found in toes, when they are used for support and locomotion only. In man we find the normal number of five (1), because his hands are used for grasping and the functions of the fingers are not the same; and (2), because man’s development was almost wholly brainward. In other respects his structure is far less specialized than most other mammals. He can not compete with carnivores in strength and ferocity, nor with herbivores in fleetness. In the struggle for life, therefore, there was nothing left for him but increase in intelligence. Probably four is the smallest number of locomotive organs consistent with highest efficiency. In retaining but two legs for locomotion, man has lost in locomotive efficiency, but by the sacrifice he liberates two limbs for higher functions.
[24] “Proceedings of American Academy of Arts and Sciences,” vol. xiv, May, 1878.
[25] Fol., “Archives des Sciences,” vol. xiv, p. 84, 1885; “Science,” vol. vi, p. 92, 1885.
[26] Of course, this is a purely imaginary case. The conditions of development of the eggs of higher animals forbid continuous watching the process. Yet we do observe in different individuals all these stages in mammals as well as other animals.
[27] These baleen plates are not modifications of teeth, as might at first be supposed, but rather of the transverse gum-ridges found on the roof of the mouth of many mammals, and conspicuous in the horse.