930 The American Naturalist. [October. 
a calcareous shell would tend to be formed as the result of the normal 
secretory activity of the oviduct, inherited from still lower forms. In 
this way alone is it possible to conceive of the evolution of the egg- 
shell of such forms of reptiles and birds. The secretory activity thus 
diverted from depositing surplus nutriment in the ovary would inevit- 
ably tend to diminish the fertility of the individual female, and starve 
the remaining ova in the ovary, unless active feeding went on during 
and after the period of the retention of the already formed ova in the 
oviduct. 
Now suppose a still further advance on this process, as a result of 
which not only the egg but the embryo is developed and nourished in 
the reproductive passages. Any further ova which are now detached 
from the ovary after fertilization has occurred under these new circum- 
stances, and as a result of copulation, cannot be fertilized, but must 
be resorbed. The growing embryo in the oviduct is also now diverting 
the whole of the spare nutriment to itself from the ovary, and thus 
tending to starve any other young ova which it may still contain. The 
consequence is that the physiological conditions established by either 
the retention of ova for an unusual period, or the viviparous develop- 
ment of the embryo through its retention in the oviduct, would 
directly tend to bring about, first of all, a diminution of fertility, and 
secondly, as a consequence of viviparity, check the future production 
of ova or germs for the time being. The result is obviously one which 
would tend to be self-perpetuating, and at the same time advantageous 
to the species. For, while the fertility of the species is diminished, 
the chances of survival are increased, so that the loss suffered in one 
direction is compensated in another. 
Maturation of the ova in the ovary, and their dehiscence in 
in forms in which the ova are ^mall, is simultaneous for very large 
numbers. On the other hand, where a large amount of yolk is added, 
besides an abundance of albumen and one or more secondary egg- 
envelopes, this simultaneity gives place to a sequence in the maturation 
of the ova, either singly, one after the other, or a seiiuence which is 
expressed in the serial arrangement of the ova in a row or rows in one 
or both oviducts. The effect of the delay of the ova in the oviducal 
passages, where they acquire additional material, must be such as to 
tend to put not only an end to simultaneity of maturation of ova 
in the ovary, but also from that very circumstance to diminish the 
absolute fertility of the species as determined by the number of ova 
matured. That the ovary, as well as testes, have been reduced in 
length and volume in the higher forais, is certain from the fact discov- 
