. The study of animal life . ut studying the detailed facts. Thesesuggest that the various types of animals have spread from Fig. 62. — Foreand hind feet ofthe horse andsome of its an-cestors, show-ing the gradualreduction inthe number ofdigits. (FromChamberss^«-cyclop.; afterMarsh.) CHAP. XVII TJie Evidences of Evolution 279 definite centres, along convenient paths of diffusion, varyinginto species after species as their range extended. But the history of the individual is even more first three grades of structure observed among livinganimals are: (i) Single cells (most Protoz
. The study of animal life . ut studying the detailed facts. Thesesuggest that the various types of animals have spread from Fig. 62. — Foreand hind feet ofthe horse andsome of its an-cestors, show-ing the gradualreduction inthe number ofdigits. (FromChamberss^«-cyclop.; afterMarsh.) CHAP. XVII TJie Evidences of Evolution 279 definite centres, along convenient paths of diffusion, varyinginto species after species as their range extended. But the history of the individual is even more first three grades of structure observed among livinganimals are: (i) Single cells (most Protozoa), (2) balls ofcells (a few Protozoa which form colonies), and (3), two-layered sacs of cells { the simplest sponges). But thesethree grades correspond to the first three steps in the indi-vidual life-history of any many-celled animal. Every onebegins as a single cell, at the presumed beginning again ;this divides into a ball of cells, the second grade of struc-ture ; the ball becomes a two-layered sac of cells. The. Fig. 63.—-Antlers of deer (1-5) in successive years; but the figure might almostrepresent at the same time the degree of evolution exhibited by the antlersof deer in successive ages. (From Chamberss Encycloji.) correspondence between the first three grades of structureand the first three chapters in the individuals life-history iscomplete. It is true as a general statement that the indi-vidual development proceeds step by step along a pathapproximately parallel to the presumed progress of therace, so far as that is traceable from the successive gradesof structure and from the records of the rocks. Even inregard to details such as the development of antlers on stagsthe parallelism of racial and individual history may beobserved. Of this correspondence it is difficult to see anyelucidation except that the individual in its life-history ingreat part re-treads the path of ancestral evolution. I have illustrated these evidences of evolution very 28o The Study
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