122 OKGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



larva, hatched at an early stage of development, laboured under ol 

 acquiring special arrangements for its protection and nourishment 

 (H. Leuckart). The proof that such relations do exist between 

 the special larval organs and the peculiar methods of nutrition and 

 protection is an important factor for the full understanding of these 

 remarkable processes, but still is by no means an explanation of them. 

 It is only by aid of the Darwinian principles and the theory of 

 descent that we can get nearer to an explanation. According to 

 this theory, the form and structure of larvae are to be considered in 

 relation to the development of the race, i.e. phylogeny, and are to be 

 derived from the various phases of structure through which the 

 latter has passed in its evolution, and in such a way that the younger 

 larval stages would correspond to the primitive, and the older, on 

 the other hand, to the more advanced and more highly organized 

 animals, w r hich have appeared later in the history of the race. In 

 this sense the developmental processes of the individual constitute a 

 more or less complete recapitulation of the developmental history cf 

 the species, complicated, however, by secondary variations due to 

 adaptation, which have been acquired in the struggle for existence * 

 (Fritz Mailer's fundamental principle, called by Haeckel the funda- 

 mental law of biogenesis). 



The greater the number of stages, therefore, through which the 

 larva passes, the more completely is the ancestral history of the 

 species preserved in the developmental history of the individual ; 

 and it is the more truly preserved the fewer the peculiarities of 

 the larva, whether independently acquired, or shifted back from 

 the later to the earlier periods of life (Copepoda.) On the other 

 hand, there are certain larval forms without any phylogenetic 

 meaning which are to be explained as having been secondarily 

 acquired by adaptation (many Insect larvae). 



The historical record preserved in the developmental history 

 becomes, however, gradually defaced by simplification and shortening 

 of the free development; for the successive phases of development 

 are gradually more and more shifted back in the life of the 

 embryo, and run their course more rapidly and in an abbreviated 

 form, under the protection of the egg membranes, and at the cost 

 of a rich supply of nutrient material (yolk, albumen, placenta). In 

 animals with a direct development, therefore, the complicated deve- 

 lopment within the egg membranes is a compressed and simplified 



* Fritz Miiller, " Fur Darwin." Leipzig 1863, p. 7581. 



