EVOLUTION IN BIOLOGY. 297 



in Bonnet's later writings; and development is merely 

 the expansion of a potential organism or " original pre- 

 formation" according to fixed laws. 



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 modifi- 

 cation 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 con- 

 verging 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, pre- 

 sented 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 mem- 

 bers 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 



