PEOPLING OF LAND AND SEA 191 



of the gills in the Vertebrates. In order that these trans- 

 formations should produce their full effect, it was necessary 

 for the eggs to become transformed, for embryogenetic 

 acceleration to intervene, and for the organs resulting from a 

 reciprocal adaptation of mother and embryo to manifest 

 themselves. It is thus impossible to contain the history of 

 organic evolution in any one of those simple formulas so dear 

 to certain philosophers — creation by the word, astral in- 

 semination, preformation, use or disuse of parts, the 

 continuity of the germ plasm, natural selection, mutations, 

 and so forth. In reality all that goes to make up the energy, 

 motion, and substance of the world has taken part in its 

 turn in the evolution of life, and its organic forms are the 

 result of the unceasing action on them of these different 

 forces whose mobility only is translated by evolution. 



Organisms themselves are capable of active intervention 

 in their own modification. When new organs appeared as a 

 result of the rapid multiplication or specialization of the 

 elements of a certain category, they might at first remain 

 unused, but favourable circumstances permitted the organism 

 to take advantage of them. The use to which it turned them 

 would then direct their subsequent modifications towards 

 some definite end. They would become more and more capable 

 of fulfilling the part that devolved upon them, and would 

 adapt themselves better and better to their function. In their 

 turn they might react on and modify the organs with which 

 they were connected. This is the histoty of the feathers of the 

 Bird and of the webbed feet of walking Vertebrates which once 

 more became aquatic. Feathers were at first merely an 

 epidermal investiture, consisting of long prominent papillae of 

 the skin, probably ramifying in all directions, as the down of 

 young birds would seem to indicate. The tegumentary papillae 

 then became imbricated, like the false scales of Serpents, and 

 consequently they tended to flatten, while at the same time 

 their mutual overlapping must have interfered with the 

 multiplication of the epidermal cells on the ventral and 

 dorsal surfaces. This multiplication then became confined to 

 the rim of the papillae, and brought about the quasi-symmetrical 

 arrangement of the branches hereditarily preserved after the 

 initial papilla was drawn back under the skin. Thus was 

 constituted the disc-shaped tegumentary appendage, made up 



