290 EVOLUTION IN BIOLOGY. [LECT. 



II. The Evolution of the Sum of Living Beings. 



The notion that all the kinds of animals and plants 

 may have come into existence by the growth and 

 modification of primordial germs is as old as speculative 

 thought ; but the modern scientific form of the doctrine 

 can be traced historically to the influence of several 

 converging lines of philosophical speculation and of 

 physical observation, none of which go farther back 

 than the seventeenth century. These are : 



1. The enunciation by Descartes of the conception 

 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 structure, 

 from extreme simplicity to very great complexity, 

 presented by living things, and of the relation of these 

 graduated forms to one another. 



3. The observation of the existence of an analogy 

 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 fundamental 

 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 developed and 



