530 EMBRYOLOGY OF THE TURTLE. Part III. 
is part and parcel of the animal system? Then come a series of changes and 
metamorphoses, at some of which one type stops, while another passes on. One 
type undergoes certain changes before it is born, another not till a longer or 
shorter time after birth; one type retains a certain peculiarity of organization 
for almost its whole lifetime, and this organization forms one of its principal 
characteristics, while im another, the same peculiarity, lasting but a short time, 
is too often looked upon as a mere scaffolding and _ prop-work, which serves 
to hold the structure in shape while it is perfecting. Because one has a long 
and the other a short existence, the same characteristics in two different animals 
are very differently estimated. Minutely described in the first case, they are per- 
haps totally ignored in the other as unessential, as having no particular reference 
to the type in which they occur. 
Can this be? Is it not true, that each and every type undergoes a series of 
changes, not only during its “embryonic period,’ but throughout life; some fol- 
lowing after longer, and others after shorter, spaces of time, so that their pecu- 
liarity and periodicity characterize this, or that, or the several different types, as 
distinct from one another? Different animals shed their teeth at diverse ages, and 
then acquire other habits. Some shed their epidermis (dandruff, scales, feathers, 
or slough) at stated periods, and others constantly. Some bear young soon after 
they are born, and others at two, three, ten, fifteen, twenty or more years of 
age, and this function ceases in them at diverse ages. And yet this latter 
change is a normal development just as truly as any which occurs at a much 
younger age. The long space of time that may follow the period of sterility 
is quite as prominent a characteristic of the life of an animal as any preceding 
state. At that period, so great a revolution takes place in the system as some- 
times to endanger life when adaptation to its requirements is accidentally pre- 
vented. Yet, after such metamorphoses, are not the peculiarities of the functions 
of some of the organs greatly changed? And so we might go on, enumerating 
many other progressions and alterations, to show, that life after birth is not fixed 
to one uniform phase; but that there is a constant and more or less frequent 
formation and suppression of functions, and a series of alterations going on in the 
organization, not only from the beginning to the end of the embryonic period, 
but ever afterwards, through the whole duration of life, till death. 
Finally, we must contend that it is a false idea of the physiology of animal 
life to suppose that in the ege the animated being is only forming; that its 
organs are only combining with each other in order to establish regular commu- 
nications between them for certain ends, and to prepare the way for a variety 
of functions, the beginning of which is not realized until a definite and unvary- 
ing relation of parts with definite proportions has been completed. As if the 
