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 

 dechne 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 httle change indicating a dechne. 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 once a year. In the 

 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, 



