84 THE EVOLUTION OF ANIMALS. 



(i.) Environmental. — Variations produced by the direct 

 action of external influences (emphasised by Buffon, 

 Treviranus, Semper, and others.) 

 (2.) Functional. — Variations produced as the results of use 

 and disuse, or by change of function (emphasised 

 by Erasmus Darwin, Lamarck, and the " Neo- 

 Lamarckians"). 

 (3.) Organismal, Constitutional, or Congenital. — Varia- 

 tions not directly traceable to (i) or (2), but resulting 

 from the inequilibrium of the protoplasm : 



or from the tendency to preponderance of anabolic 



or katabolic processes (Geddes) : 

 or from pathological change (Sutton) : 

 or from the internal complexity of the organism, 

 the " struggle of parts within the organism " 

 (Roux) : 

 or from the predominance of an accessory over 



a principal function (Dohrn) : 

 or from the rhythm between vegetative and re- 

 productive preponderance (Geddes) : 

 or from the stimulus that one part gives to another 



(Kleinenberg) : 

 or from the tendency to " cumulative growth " in 



definite directions (Eimer) : 

 or from the intermingling of two different living 

 plasmas in fertilisation (Weismann). 

 Congenital variations are regarded by most Darwinians as 

 numerous, indefinite, arbitrary, " fortuitous," " spontaneous," 

 without any necessary connection with the experience, habits, 

 needs, or external surroundings of the animal. 



According to a minority, variations occur in a few definite 

 directions, necessarily determined by the constitution, habits, 

 and surroundings of the organism or of its ancestors. 



B. The Secondary Factors in Evolution, as the result of 

 which variations become new, well-adapted, and pro- 

 gressive species. 



(i.) Natural Selection. — Though Charles Darwin believed 

 in the efficacy of external influences and of use and disuse 

 as factors in evolution, the distinctive contribution which he 



