202 CLOSE OF THE PRE-EYOEUTIONARY PERIOD 



in the front end to form a head all in direct adaptation 

 to this forward movement ; they make up the vast majority 

 of animals. 



The fundamental forms of living things are, however, 

 merely so many themes on which a multitude of further 

 variations are woven, through the action of the laws which 

 rule the detail of organic diversities. These further laws 

 may be set down under four main heads. Under the first 

 comes the law of the existence of certain fundamentally 

 distinct structural types, which are distinguished from one 

 another by their ground-form, by the number of organ- 

 systems, and by the number of homotypic organs they possess, 

 but principally by the relative position of the organs to one 

 another (principle of connections). The form and connections 

 of the nervous system are of particular importance in 

 distinguishing the types (cf. Cuvier). The second factor in 

 the diversity of organic form is the action of certain laws 

 of progressive development 1 (Entivickclmigsgesetze}, which 

 bear the same relation to the development of the animal 

 kingdom as the laws of individual development bear to the 

 development of the embryo, for organs appear in the 

 different animal series in much the same order and 

 manner as they develop in the individual. These laws are 



(1) progressive differentiation of functions and organs; 



(2) numerical reduction of serially repeated parts ; (3) 

 concentration of functions and their organs in particular 

 parts of the body; (4) centralisation of organ-systems 

 and parts of such, so that they come to depend upon one 

 central organ; (5) internalisation of the "noblest" organs, 

 unless these are necessarily external, and (6) increase in 

 size of the whole or of parts. Of these the law of differ- 

 entiation is by far the most important, and most of the 

 others arc in a sense merely special cases of this fundamental 

 law. To this law of differentiation is due the increase in 

 complexity or perfection of organisation which is shown by 

 all the animal series. Hronn himself recognised the great 

 similarity of this law of progressive differentiation to Milne- 

 Edwards' principle of the division of labour; he seems, how- 

 ever, to have arrived at it independently. 



1 Cf. Meckel's Principle of progressive Evolution, su/v, p. 93. 



