1886, | Numerical variations in Animals. IOft 
same animals out of all proportion to its ordinary census of occu- 
pation, and in another season pass, by an abrupt change or per- 
haps through a series of less violent alternations, to a condition 
of comparative or actual denudation of these residents. The 
work of the Fish Commission has made us familiar with facts of 
much wider import, when large sections of the oceanic basin 
have become depopulated. This latter case appears to be catas- 
trophic in its causes, but comes within the scope of our sugges- 
tion as to the fluctuating fertility of a species. And the fact be- 
comes sometimes apparent that in a restricted region command- 
ing amore or less fixed supply of nourishment the size of the 
animals will increase in the years of decreased fertility, and cor- 
respondingly diminish in the seasons of enhanced productivity, a 
relation not unnatural. 
To what extent we may parallelize these two classes of facts, 
the one dealing with the changing abundance of fossil shells or 
remains, either vertically or horizontally distributed in beds of 
the same age, and the other exhibiting the varying numbers, in 
Separated seasons, of contemporaneous animals along our sea- 
| boards, or in our fresh-water lakes, or even, so far as we can de- 
termine, in the pelagic areas, is not at first, or in all cases equally 
easy to determine. But it is possible to review some considera- 
tions bearing upon the general question. _ 
The observations which may be adduced as bearing on this 
question are necessarily widely scattered, and when found are for 
the most part concerned with those forms of life which subserve 
Some industrial or economic uses, or with those in close relation 
with the former, as, for instance, the recorded irruptions of star- 
fishes (Asterias forbesit) and “ drills” (Urosalpinx cinerea) in dif- 
ferent years upon our oyster beds. In classifying, however, the 
efficient causes which effect these variations of animal populous- 
ness, without entrenching upon ground more or less speculative, 
we may say that the changing abundance of animal forms in 
different years or localities arises mainly from: 
Ist. Opportunity for or difficulty in obtaining fecundation. 
2d. Constitution, rate of growth, habits, etc., of organism. 
3d. Character of habitat in relation to bottom. ; 
4th. Phenomenal influences, as cataclysms, poisoned or heated 
waters, storms, destruction by enemies. 
I. Opportunity for or difficulty in obtaining fecundation—That 
