DEVELOPMENT. 31 



instances, by gemmation from, or division of, the parent-being ; 

 but this process is an exhaustive one, and cannot be carried 

 out indefinitely ; when, therefore, it is necessary to insure the 

 continuance of the species, the sexes must present themselves, 

 and the germ and sperm must be allowed to come in contact 

 with one another.' 



It should be added that the act of sexual reproduction, 

 though it insures the perpetuation of the species, is very de- 

 structive to the life of the individual. The formation of the 

 essential elements of reproduction appears to be one of the 

 highest physiological acts of which the organism is capable, 

 and it is attended with a corresponding strain upon the vital 

 energies. In no case is this more strikingly exhibited than in 

 the majority of insects, which pass the greater portion of their 

 existence in a sexually immature condition, and die almost 

 immediately after they have become sexually perfect, and 

 Lave consummated the act whereby the perpetuation of the 

 species is secured. 



11. DEVELOPMENT, TRANSFORMATION, AND METAMORPHOSIS. 



Development is the general term applied to all those changes 

 which a germ undergoes before it assumes the characters 

 of the perfect individual ; and the chief differences which are 

 observed in the process as it occurs in different animals con- 

 sist simply in the extent to which these changes are external 

 and visible, or are more or less completely concealed from 

 view. For these differences the terms ' transformation ' and 

 ' metamorphosis ' are employed ; but they must be regarded 

 as essentially nothing more than variations of development. 



Transformation^ the term employed by Quatrefages to desig- 

 nate ' the series of changes which every germ undergoes in 

 reaching the embryonic condition; those which we observe 

 in every creature still within the egg ; those, finally, which 

 the species born in an imperfectly developed state present in 

 the course of their external life.' 



Metamorphosis is defined by the same author as including 

 the alterations which are ' undergone after exclusion from the 

 egg, and which alter extensively the general form and mode of 

 life of the individual.' 



Though by no means faultless, these terms are sufficiently 

 convenient, if it be remembered that they are merely modifi- 

 cations of development, and express differences of degree and 

 not of kind. An insect, such as a butterfly, is the best illus- 

 tration of what is meant by these terms. All the changes 

 which are undergone by a butterfly in passing from the fe- 

 cundated ovum to the condition of an imago, or perfect insect, 



