July, 1891. 
ANIMAL PEDIGREES. 
147 
struggle for existence by producing the maximum number of 
young, to others it is of greater importance that the young- 
on hatching should be of considerable size and strength, and 
so better able to begin the world on their own account. In 
other words, some animals may gain by producing a large 
number of small eggs, others by producing a smaller number 
of eggs of larger size— i.e., provided with more food yolk. 
The immediate effect of a large amount of food yolk is to 
mechanically retard the processes of development; the ultimate 
result is to greatly shorten the time occupied by development. 
This apparent paradox is readily explained. A small egg, 
such as that of Amphioxus, starts its development rapidly, 
and in about eighteen hours gives rise to a free swimming 
larva, capable of independent existence, with a digestive 
cavity and nervous system already formed ; while a large 
egg, like that of the hen, hampered by the great mass of food 
yolk with which it is distended, has, in the same time, made 
but very slight progress. 
From this time, however, other considerations begin to 
tell. Amphioxus has been able to make this rapid start 
owing to its relative freedom from food yolk. This freedom 
now becomes a retarding influence, for the larva, containing 
within itself but a very scanty supply of nutriment, must devote 
much of its energies to hunting for, and to digesting its food, 
and hence its further development will proceed more slowly. 
The chick embryo, on the other hand, has an abundant 
supply of food in the egg itself; it has no occasion to spend 
time searching for food, but can devote its whole energies to 
the further stages of its development. Hence, except in the 
earliest stages, the chick develops more rapidly than Am¬ 
phioxus, and attains its adult form in a much shorter time. 
The tendency of abundant food yolk to lead to shortening 
or abbreviation of the ancestral history, and even to the 
entire omission of important stages, is well known. The 
embryo of forms well provided with yolk takes short cuts in 
its development, jumps from branch to branch of its genea¬ 
logical tree, instead of climbing steadily upwards. 
An excellent illustration of the influence of food yolk on 
development is afforded by the life histories of frogs. The 
common frog, Rana temporaria , lays, as is well known, eggs 
of small size, about ^ inch in diameter. A small egg 
can only contain a limited amount of food yolk, and hence 
the young frog can only accomplish a small part of its develop¬ 
mental history within the egg, and must then hatch in order 
to obtain food from without. Consequently, a frog hatches 
not as a frog but as a tadpole, i.e., at the fish stage in the 
