4 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 
_yery different, likewise exhibit modifications .of a 
common plan. 
_ 5. The observation of the existence of structures, 
- ina 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. 
