296 THE PHILOSOPHY OF BIOLOGY 



syntheses as are effected by the plant. Even if the 

 Annehd worm, the Arthropod, and the Vertebrate were, 

 at the origin of their ancestries, animals which were 

 very like each other in the morphological sense ; even 

 if there are some Arthropods which are very like 

 Annelids, and some Annelids which might very easily 

 be imagined to become transformed into Vertebrates, 

 and some extinct Arthropods which may after all have 

 been Vertebrates, yet it is the case that the tendencies 

 of the evolution of each of these groups have been very 

 different. All the while the Vertebrate tended more 

 and more to develop a rigid axial rod or notochord, 

 becoming later a jointed vertebral column, and a soft, 

 pliable, exo-skeleton ; while the Arthropod tended more 

 and more to develop a rigid exo-skeleton, and to remain 

 soft in its axial parts. Even if these two tendencies 

 may not have been fully realised, is it not the case that 

 they are really different things ? The evolutionary 

 process has therefore been, in its essence, the develop- 

 ment, or unfolding, of tendencies originally one. 



What is the evolutionary process ? It is usually 

 regarded as a progress from organic simplicity towards 

 organic complexity. Yet if we think about it as a 

 physical process we cannot say that any one stage is 

 any more simple or complex than any other stage. 

 Let us compare organic evolution with the process of 

 inorganic evolution, as, of course, we are compelled to 

 do if we regard the former process as a physico-chemical 

 one. Assume, then, that the nebular hypothesis of 

 Kant and Laplace is true — it will make no difference 

 to our argument even if this hypothesis is not true, 

 and it is more easily understood than any other hypo- 

 thesis of planetary evolution. Originally all the 

 materials composing our solar system existed in the 

 form of a gaseous nebula possessing a slow rotatory 



