RETROSPECT AND PROSPECT 277 



Discontinuity is found chiefly in those characters in which 

 a continuous mode of change is impossible. As to the physico- 

 chemical constitution of animals and plants it has been well 

 said that there can be no continuity between two distinct 

 chemical formulas, or in many physicochemical functions and 

 reactions. There are also certain form and proportion char- 

 acters in which continuity is impossible — for example, the 

 sudden addition of a new tooth to the jaw, or of a new verte- 

 bra to the backbone. 



From these well-ascertained facts of the sudden or salta- 

 tory appearance of characters, some have rashly inferred 

 that there can be no continuity between species, whereas it 

 is now known in mammalogy, in palaeontology, and to a less 

 extent in ornithology that a large number of so-called species 

 in nature show a complete continuity. Although the part 

 which sudden changes or "saltations" from character to char- 

 acter play in experimental evolution and artificial selection 

 is very prominent, it remains to be seen how large a part they 

 play under natural conditions. 



We realize that it is far more dif!icult to ascertain the causes 

 of such continuous independent and more or less orderly and 

 adaptive evolution of single characters than to comprehend 

 evolution as Darwin's adherents of the present day imagine it 

 to be, namely fortuitous and saltatory, for it is incumbent upon 

 us to discover the cause of the orderly origin of every single 

 character. The nature of such a law we cannot even dream 

 of at present, for the causes of the majority of vertebrate adap- 

 tations remain wholly unknown. 



Negatively we may say from palaeontology that there is 

 positive disproof of the existence of an internal perfecting 

 principle or entelechy of any kind which would impel animals 

 to evolve in a given direction regardless of the direct, reversed, 



