146 SECRETS OF ANIMAL LIFE 
and in consequence the period of embryonic de- 
velopment has been relatively shortened or con- 
densed. The cock that crows in the morn is, on 
the whole, except in brain, as complex a creature 
as the cow that tosses the dog, but the chick breaks 
its way out of the egg after three weeks of incuba- 
tion, whereas the calf is carried by the cow for 
about nine months of antenatal life. In the 
contrast between the callow nestling type, which 
corresponds to prolonged infancy, and the preco- 
cious chick type, which corresponds to abbreviated 
infancy, we have another familiar illustration of the 
elasticity of the curve of life. How striking is the 
case of the Mound-Builders, where the mother-birds 
have evaded brooding and where the young, hatched 
within the heap of fermenting vegetation, are able 
to fly right away. The great majority of fishes are 
oviparous, with both embryonic and larval phases 
in the water. In this there are great risks and the 
juvenile mortality is enormous. Yet, as there are 
plenty of fry to spare, most species of modern fishes 
continue to prosper. But how vivid is the contrast 
between their life-curve and that of the whale, with 
a single calf slowly developing within the mother 
and born into the sea a fully-formed and richly 
endowed cetacean. Just as there are plants which 
remain for life like half-opened buds, and others 
which flower before they leaf, and so forth, so there 
are animals which have a long youth and others a 
long maturity, some that are born old and others 
that die young; some which break down suddenly 
