vi EVOLUTION IN BIOLOGY 205 



1. The enunciation by Descartes of the concep 

 tion that the physical universe, whether living or 

 not living, is a mechanism, and that, as such, it is 

 explicable on physical principles. 



2. The observation of the gradations of struc 

 ture, from extreme simplicity to very great com 

 plexity, presented by living things, and of the 

 relation of these graduated forms to one another. 



3. The observation of the existence of an anal 

 ogy between the series of gradations presented by 

 the species which compose any great group of 

 animals or plants, and the series of embryonic 

 conditions of the highest members of that group. 



4. The observation that large groups of species 

 of widely different habits present the same funda 

 mental plan of structure ; and that parts of the 

 same animal or 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 devel 

 oped and have definite functions in other species 

 of the same group. 



6. The observation of the effects of varying 

 conditions in modifying living organisms. 



7. The observation of the facts of geographical 

 distribution. 



8. The observation of the facts of the geological 

 succession of the forms of life. 



