90 ESSAY ON CLASSIFICATION 



In others again, which also acquire within certain limits a definite 

 size, the Mammalia, for instance, the growth is slower in early life, 

 and maturity is attained as in man at an age which forms a much 

 longer part of the whole duration of life. 



In Insects the period of maturity is, on the contrary, generally 

 the shortest, while the growth of the larva may be very slow, or at 

 least that stage of development lasts for a much longer time than the 

 life of the perfect Insects. There is no more striking example of this 

 peculiar mode of growth than the seventeen-years' locust, so fully 

 traced by Miss M. H. Morris. ^^^ 



While all longlived animals continue as a matter of course their 

 existence through a series of years under the varying influence of 

 successive seasons, there are many others which are periodical in 

 their appearance; this is the case with most insects, but perhaps in a 

 still more striking manner with Medusas. 



The most interesting point in this subject is yet the change of 

 character which takes place in the different stages of growth of one 

 and the same animal. Neither Vertebrata, nor Mollusks, nor even 

 Radiata exhibit in this respect anything so remarkable in the con- 

 tinuous changes which an individual animal may undergo as the 

 Insects, and among them those with so-called complete metamor- 

 phoses, in which the young (the larva) may be an active, wormlike, 

 voracious, even carnivorous being, which in middle life (the chrysalis) 

 becomes a mummylike, almost motionless maggot, incapable of tak- 

 ing food, ending life as a winged and active insect. Some of these 

 larvae may be aquatic and very voracious, when the perfect insect 

 is aerial and takes no food at all. 



Is there any thing in this regulation of the duration of life in ani- 

 mals which recalls the agency of physical forces? Does not, on the 

 contrary, the fact that while some animals are periodical and bound 

 to the seasons in their appearance, others are independent of the 

 course of the year, show distinctly their independence of all those 

 influences which under a common expression are called physical 

 causes? Is this not further illustrated in the most startling manner 

 by the extraordinary changes, above alluded to, which one and the 

 same animal may undergo during different periods of its life? Does 

 this not prove directly the immediate intervention of a power 



"®See Harris, Insects Injurious to Vegetation, p. 184. 



