2590 ANIMAL ACTIVITIES. 
slits in the embryos of almost all the Chordata suggests 
a kinship, through inheritance, among the group. 
Distribution. We have noted that aquatic insects 
breathe air by using organs like those of terrestrial 
insects. From this fact it is commonly inferred that 
the ancestors of such insects were terrestrial or aerial. 
Animals which have wandered into a new environment 
have sometimes so changed their mode of life and 
their structure as to form a new species differing from 
the ancestral forms common to the old locality. In 
cases like these relationships could not be detected by 
structure alone. Hence the facts of distribution must 
be reckoned with in determining kinship. 
Mimicry and Protective Devices. As we have 
seen, the real characteristics of an animal may be 
obscured by the devices it has adopted for its better 
protection; hence these devices, too, must be con- 
sidered in connection with relationships. 
Heredity. We expect the young of an animal to 
grow into a form like that of its parent. Every egg of 
any kind of animal goes through a definite course of 
development which is essentially like that of every 
other egg of the same species. Eggs of two different 
species may develop exactly alike for a time, but they 
diverge as development goes on. Each egg is true to 
its kind. This fact of the continual repetition of 
ancestral traits we call heredity. Experience shows 
that there are limits to heredity. The young is the 
product of two parents, and these are not exactly alike. 
Young animals from the same parents differ in many 
ways. In any case, however, the differences do not 
impress us much. With heredity it is the resemblance 
to ancestors which is the striking fact, and this resem- 
blance is of great assistance in tracing an animal’s 
ancestry. 
Variation. Divergences from the regular path of 
heredity are called variations. For the most part we 
do not know the causes of variations. That such var- 
