[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. [28] “Proceedings of the California Academy of Science,” vol. v, p. 152. 1873. [29] For fuller discussion of this subject, see “Bulletin of the California Academy of Science,” No. 8, 1887, and “American Journal of Science,” for Dec., 1887. [30] “Origin of Races of the Dog.” “Annals and Magazine of Natural History,” vol. xvii, p. 295. 1886. [31] Mr. Galton (“Nature,” August 26, 1886) has used a diagram similar to the above (which I first used in 1879) to illustrate the law of sexual attraction and repugnance. [32] This subject is more fully discussed by the author in an article entitled “Genesis of Sex,” in “The Popular Science Monthly,” vol. xvi, p. 167, 1879.