INTRODUCTORY PROBLEMS 13 



youthful period and a very short period of maturity, as in the 

 case of may-flies or ephemerids, many of which spend two or 

 three years as larvae in the water, and not more than two or 

 three days as adults in the air. Or conversely, there may be 

 a rapid development and a short youth, followed by a long period 

 of maturity, as in the case of many birds and mammals. In 

 other cases, as in most fishes, there is no definite limit of growth, 

 and the animal goes on for years steadily growing larger and larger. 



Long adolescence. Short maturity. Death 

 soon after maturity. 



B 



Short adolescence. Long period of 

 maturity. Gradual senescence. 



Minor waves on general life curve. 



Seasonal curve. 



FIG. 2. 



We suggest this as a graphic method, and leave the idea to 

 be elaborated. It should be pointed out (i) that a similar 

 curve may be used to express the year's life of an animal, summer 

 being the period of maximum vigour at the crest of the curve ; 

 (2) that when the animal lives for many years the fluctuations 

 of its rate of growth, etc. (literally registered in the case of 

 fishes in the lines on the scales) may be expressed by minor waves 

 on the general curve (as in Fig. 2, C.) ; (3) that very few wild 

 animals show any traces of degeneracy even when they live for 

 many years, they have a senescence without senility; and (4) 

 that it is important to distinguish violent death (which befalls 

 such a large proportion of wild animals) from microbic death due 

 to poisoning by parasites, and from natural death due to the 

 accumulation of the effects of wear and tear. 



SECTION 5. RESEMBLANCES AND DIFFERENCES BETWEEN 

 PLANTS AND ANIMALS. (a) Resemblances in Function. That the 



