892 The American Naturalist. [October, 
is known as atavism. As the influence mentioned is a varia- 
tion in growth force, atavism seems due to a check in develop- 
ment, the organism not attaining its full unfoldment. Atavism 
is usually considered as applying to the whole organism, but 
it may confine its action to certain parts of the organism while 
the others attain full development, thus producing conditions 
whose atavistic origin is not evident, and which are accepted 
as results of ordinary variation. 
Two conditions are probably concerned in atavism, one 
being deficiency of nutriment, the other the influence of en- 
vironment. In truth, there is good reason to believe that two 
parallel, and, to a certain extent, mutually exclusive, processes 
are at work in the organism—those of growth and develop- 
ment. The developmental powers only proceed actively under 
certain conditions. They differ from growth, which is simply 
increase of tissue, in being changes of tissue, due to chemical 
or other influence, and set in train by inherent tendencies in 
the organism. 
There are abundant evidences that energetic nutrition acts as 
a hindrance to development, and yet is preliminarily necessary 
to it. The two cannot be active at the same time. While 
nutrition is active, development is latent, and it cannot set in 
actively without a marked cessation of nutritive energy. Yet 
it must be preceded by a period of nutritive activity to provide 
the tissue within which the developmental forces act, and in 
which a degree of chemical reduction would seem to precede or 
accompany the re-organization of tissue into new forms. If the 
preliminary nutrition be wanting, development may be slight 
and imperfect, or not appear at all, through lack of the quan- 
tity of tissue necessary to the changes in organization. 
As regards development, or rearrangement of organic tissue, 
a question arises as to what influences set it in operation, so that, 
at fixed intervals, nutrition is checked, growth ceases, and ac- 
tive organic change sets in. Inherent tendencies to such change 
seem to exist in the tissues, their molecular constitution being 
such that a series of successive rearrangements take place, re- 
producing conditions which successively appeared in the 
phylogenetic evolution of the form, and were gone through 
