210 THE GENESIS OF SPECIES. [On. VIII. 



of symmetry of growth, just like the equally beautiful and 

 somewhat similar forms of the compound six-rayed, star- 

 shaped crystals of snow." 



Altogether, then, it appears that each organism has an 

 innate tendency to develop in a symmetrical manner, and 

 that this tendency is controlled and subordinated by the 

 action of external conditions, and not that this symmetry is 

 superinduced only ab externo. In fact, that each organism 

 has its own internal and special laws of growth and 

 development. 



If, then, it is still necessary to conceive an internal law 

 or "substantial form," moulding each organic being and 

 directing its development as a crystal is built up, only in 

 an indefinitely more complex manner, it is congruous to 

 imagine the existence of some internal law accounting 

 at the same time for specific divergence as well as for 

 specific identity. 



A principle regulating the successive evolution of dif- 

 ferent organic forms is not one whit more mysterious 

 than is the mysterious power by which a particle of struc- 

 tureless sarcode develops successively into an egg, a grub, 

 a chrysalis, a butterfly, all the conditions, cosmical, phy- 

 sical, chemical, and vital, being supplied which are the 

 requisite accompaniments to determine such evolution. 



