246 Experimental Zoology 
the same for both, On the other hand, rabbits attain a larger size 
than guinea pigs, not because they grow for a longer time, but 
because they grow faster. Thus there are two different ways 
of attaining larger size. 
Senescence 
After reaching a certain stage in their growth some organisms 
begin to “grow old.” There is no very fixed period at which the 
decline may be said to begin, for after reaching full growth 
there may follow a relatively long period before any evidence of 
growing old can be detected. Thus in man the full growth is 
reached about the twenty-fifth year, but for the following ten 
years or more there may be little change indicating a decline. In 
many insects, on the other hand, the complete growth coincides 
very nearly with sexual maturity, and after the eggs of the 
female are laid the decline may follow very. quickly; in fact, in 
some cases death follows at once, so that there is no period 
of senescence at all. 
In some of the crustaceans, there are species in which, 
apparently, the individuals grow larger as long as they live, as 
seen in the lobster and to a less extent in crabs and crayfishes. 
Some mollusca also seem to continue to grow for many years, 
adding each year a new and larger edge to the shell. In most 
of these forms the period of egg laying occurs oncea year. Inthe 
vertebrates we find that fishes and some species of amphibians 
and reptiles grow continuously, although very slowly, after a 
certain size has been reached —so slowly, indeed, that they may 
be said to have an upper limit of growth. On the other hand, 
birds and mammals cease to grow after a certain size has been 
reached that may be quickly attained. 
Minot looks upon senescence as due to the loss of power to 
grow. He thinks that previous writers have given an incorrect 
interpretation of the rate of growth. They count the absolute 
increments of equal successive periods, but during each period 
the size of the body increases, and this should be considered. 
Under these conditions if the rate of growth were constant, 
