298 EVOLUTION IN BIOLOGY. 



plant, the functions of which are very different, likewise 

 exhibit modifications of a common plan. 



5. The observation, of the existence of structures, in a 

 rudimentary and apparently useless condition, in one 

 species of a group, which are fully developed and have 

 definite functions in other species of the same group. 



6. The observation of the effects of varying condi- 

 tions in modifying living organisms. 



7. The observation of the facts of geographical dis- 

 tribution. 



8. The observation of the facts of the geological suc- 

 cession of the forms of life. 



1. Notwithstanding the elaborate disguise which fear 

 of the powers that were led Descartes to throw over his 

 real opinions, it is impossible to read the " Principes de 

 la Philosophic " without acquiring the conviction that 

 this great philosopher held that the physical world and 

 all things in it, whether living or not living, have origin- 

 ated by a process of evolution, due to the continuous 

 operation of purely physical causes, out of a primitive 

 relatively formless matter.* 



The following passage is especially instructive : 



" Et tant s'en faut que jo veuille que 1'on croie toutes les choses 

 quo j'ecrirai, que meine je pretends en proposer ici quelques unes 

 que je crois absolument etre fausses ; a savoir, je ne doute point que 

 le monde n'ait ete cr66 au commencement avec autant de perfection 



* As Buffonhas well said : " L'idee de raraener 1'explication de tous les 

 ph6nomenes a des principcs mecaniques cst assurement grande et belle, ce 

 pas est le plus hardi qu'on peut faire en philosophic, et c'est Descartes qui 

 1'a fait."?, c. p. 50. 



