153 MEASTINtt OF THE STSTEil. 



that of the adult ; and we can thus understand how larvae of many 

 insects belonging to different orders can present great resemblances 

 to one another and be unlike the larvae of insects of the same order. 

 While as a general rule the development of the individual is an 

 advance from a simpler and lower organization to one more complex 

 which has become more perfect by a continued division of labour 

 among its parts and we shall later find a parallel to this law of 

 perfection of the individual in the great law of progressive perfection 

 in the development of groups yet the course of development may, 

 in particular cases, lead to numerous retrogressions, so that we may 

 find the adult animal to be of lower organization than the larva. 

 This phenomenon, which is known as retrogressive metamorphosis 

 (Cirripedia and parasitic Crustacea), corresponds to the demands of 

 the selection theory, since under more simple conditions of life, where 

 nourishment is more easily obtained (parasitism), degradation and 

 even the loss of parts may be of advantage to the organism. 



Again, the facts of embryonic development, when considered in 

 relation to the gradations expressed in the system are in complete 

 accord with the theory of evolution. Numerous examples may be 

 cited to prove that features, not only of the simple and more 

 primitive, but also of the more perfectly organised groups of the 

 same type, are reflected in the successive phases of fo3tal life. In 

 the case of a complicated free development by metamorphosis, which 

 is usually correlated with an unusual simplification of the fcetal 

 development within the egg- membranes, the relation of the successive 

 larval stages to the allied smaller groups of the system, to the 

 genera, families and orders, is more direct and striking. For example, 

 in the early stages of the embryonic development of mammals certain 

 structures occur, which in the lower fishes endure throughout life. 

 Later stages show peculiarities which correspond to the characters of 

 amphibia. The metamorphosis of the frog begins with a stage which 

 in form and organization and mode of locomotion agrees with the fish 

 type ; and this stage is succeeded by numerous other larval stages 

 in which the characters of the other orders of Amphibia (Perenni- 

 branchiata, Salamandrinidse) and of individual families and genera of 

 the same are repeated. 



This undeniable likeness between the successive stages of individual 

 development and between allied groups of the system allows us to 

 institute a parallel between the former and the evolution of the 

 species. The evolution of the species finds, it is true, a most imper- 

 fect expression in the relationship of the systematic groups, and can 



