174 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
a 
that it should be a perfect repetition of phylogeny; very much of the 
long story must of necessity be omitted. 
2. Through all the stages of development the embryo must be 
rendered able to live and grow and thrive through adaptation to its 
surroundings and changes in its environment. In some animals 
development takes place within the body of the mother; in others the 
embryo is protected by the hard egg-shell, as in birds, while the eggs 
of certain fishes and many invertebrates float freely in the sea and are 
almost without protection. Such differences in environment necessi- 
tate differences in the mode of development, while the presence or 
absence of a Jarge amount of inert food-material, or yolk, exerts a great 
influence in determining the steps of ontogeny. 
3. Many animals pass through a larval stage of development, in 
which the immature young leads an independent and self-sustaining 
existence, during which it is very different in appearance and structure 
from its adult parents. Familiar instances of this mode of develop- 
‘ment are to be found in the tadpole, which is the larva of the frog, and 
the caterpillar, the larva of a butterfly. Larvae are fully subject to 
the struggle for existence and must adapt themselves to their environ- 
ment and to changes in that environment, exactly as do adults, if they 
are to survive. In this way many changes are introduced into the 
ontogeny which can have no phylogenetic significance. It is found in 
several known instances, that nearly allied species, living under 
different conditions, have quite different modes of ontogeny, though 
their ancestral history must have been substantially identical. In one 
and the same species of marine worms, for example, which inhabits 
both the warm Mediterranean and the cold waters of the North Sea, 
the larva of the northern form is quite distinct from that of the 
southern. In attempting to interpret the meaning of embryological 
facts, it is thus necessary to distinguish sharply between those features 
which are derived from a long inheritance, and are therefore called 
palingenetic, from those which have been secondarily introduced in 
response to the changing needs of embryonic or larval life. These 
secondary features are termed cenogenetic. 
“Tf we are compelled to admit that cenogenetic characters are 
intermingled with palingenetic, then we cannot regard ontogeny as a 
pure source of evidence regarding phyletic relationships. Ontogeny 
accordingly becomes a field in which an active imagination has full 
scope for its dangerous play, but in which positive results are by no 
means everywhere to be obtained. To attain such results, the palin- 
