76 PRIMITIVE FORMS OF LIFE 



verifying the laws deduced from a strict comparison of living 

 forms and from the careful study of the possible influence 

 exerted upon them by exterior environment, either in their 

 adult forms or in the course of embryonic development. 



The laws thus formulated have as rigorous and as absolute 

 a character as those which control physical and chemical 

 phenomena. After they have once been established in a 

 definitive manner, the past history of each one of the main 

 groups of living organisms can be reconstructed and the various 

 phases of this past linked to preceding causes. Thus we may 

 eliminate the valueless hypotheses and illusory philosophical 

 conceptions which have so long concealed the true explanation 

 of facts under false principles purporting to be axioms. Such 

 erroneous notions are to be found in Leibnitz's principle of 

 continuity — the Natura non facit s alius of Linnaeus — Geoffroy 

 Saint-Hilaire's law of unity in the plan of organic form in the 

 animal kingdom, Cuvier's theory of unity, even as limited to 

 the main branches, Charles Bonnet's " ladder of being ", 

 Blainville's Degeneration of Types, etc. These premature 

 conclusions based on limited observation, satisfied human 

 reason only at a time when science could not pretend to explain 

 the nature of living organisms, in the proper sense of the term. 



Darwin x made admirable use of instinct and of sexual 

 selection to explain the preservation, diffusion and even perhaps 

 the exaggeration of useful characters. He could offer an explana- 

 tion of the adaptation of animals and plants to the conditions of 

 their existence, an adaptation so close that it gave rise to the 

 notion of their predestination for these. He could explain the 

 splitting up of the zoological and botanical series into species 

 separated by seemingly unbridgable gaps. But his theories did 

 not go so far as to determine the causes that made the 

 distinctive characters appear. 



Darwin did not even broach the problem of the significance 

 of structural types as they are now called, in either the vegetable 

 or animal kingdom. Later on Weismann 2 assigned a mysterious 

 function, in the evolution of organisms, to the living sub- 

 stance which mainly constituted the reproductory cells for 

 which he claimed a constitution different from that of the 

 body-cells. This substance he called the germen, or germ 



1 XXIV and XXV. * XXVI. 



