974 The American Naturalist. [November, 
proverb and song. All these outward changes are certainly 
expressions of inner changes equally important and significant. 
In early life the vegetative functions preponderate. Eating and 
attendant growth are the all-important activities. But bye and 
bye the animal functions pf motion come into play. Nutrition 
then ceases to be the dominant function, and the surfeited tissues 
give up their useless store to the growing demands of higher 
activities. The result is a redistribution to suit the specific needs 
of the animal organization in question. This redistribution does 
not cease until a new equilibrium is established which harmonizes 
with the existing orders of activity. Then a stage of equilibrium 
ensues, which may be called the balance of middle life. It con- 
tinues until the already abridged functions of nutrition begin to 
yield further ground, weakened by the approaching exhaustion of 
an ebbing and dwindling vitality. This waning of the nutritive 
functions is manifested by a loss of flesh, and the angular, bony 
framework projects out through the wasted habiliments of mus- 
cular and adipose tissue. At last the vital store is exhausted, the 
nutritive processes cease, and the animal dies. Such, in brief, is 
the story of the average mammal’s life, though the details are 
often disguised in various ways. While we cannot exactly for- 
mulate the variations of tissue at different periods, yet an average 
of many weighings will not lead us much astray. Such averages 
of different species closely allied may be compared with profit. 
We are all more or less familiar with certain differences of abso- 
lute and relative weight and volume in the homologous tissues of 
different species. Results of this sort, especially on the brains of 
animals, are quite frequently met with. In all such cases, how- 
ever, we are often perplexed to know, especially in animals 
with only remote systematic affinities, just how far it is legitimate 
to compare apparently homologous organs, and where such com- 
parisons should end. For this reason it is generally well to 
restrict our comparison to animals of the same subkingdom, and, 
better still, of the same class, for the greater the divergence of 
remotely analogous tissues the less reliance can be placed on the 
comparative results. There are other difficulties which arise in 
~ the study of the quantities of tissue in the same animal. In 

