“ 
978 The American Naturalist. [November, 
formed into some higher psychical energy. Finally, there are 
some other tissues’ the quantity of which does not seem to be 
subject to the limitative adjustments of other tissues, because they 
are not directly concerned in the vital economy, and they fluc- 
tuate very perceptibly with the changing conditions of the ani- 
mal’s environment. Such is the adipose tissue. Cells of fat may 
increase independently of any mechanical laws prescribed by 
other tissues (of course within limits compatible with life), as in 
the omentum of corpulent people and the layers of adipose tissue 
in swine. We thus see that there are quite well-defined types of 
growth among the tissues of animals. There are those which 
increase and decrease together, those which increase at the expense 
of others, and those which vary independently. Of course these 
lines are not absolutely distinct, and probably every tissue in 
some degree embodies them all. This classification is given as 
having a general value only; but while there are specific and indi- 
vidual variations which have no apparent connection with any 
general evolutionary principles, there are variations, extending 
over longer periods of time, which have a historical value to the 
anatomist and paleontologist. They are the larger cycles of 
change in which these individual cycles move and exist. In the 
brief period of a human lifetime they are disguised by the seem- 
ingly irrelevant fluctations of the present, and it is only in the light 
of a remoter history that we gain a full conspectus of the progress 
of these events. 
All through the Paleozoic and Mesozoic ages there was a remark- 
able profusion of moll and radiate life. Lime-secreting animals 
were the prevailing type in all seas, and the seas were everywhere. 
In many places the rocks are almost entirely composed of the 
remains of these animals. Finally fishes made their appearance. 
They continued to develop, and have reached their highest phase of 
specialization at the present time. Then the reptiles came, and at 
length the mammals. This succession has been marked by a 
concomitant succession in the relative quantities of animal tissues. 
When the primeval seas were surcharged with carbonate of lime 

~ the organisms then living rapidly used up the excess in secreting - 
: poe coverings for their softer he in some of the = 
3 

