362 Studies in the Theory of Descent. 



production of new characters than natural selection, 

 which must always, in the first place, operate upon 

 the results of this physical nature, i.e. upon the 

 variations presented, and can thus call new ones 

 into existence. 



It requires but a slight alteration of the defini- 

 tion to make out of this " restricted " or " limited 

 variability," which is the necessary consequence of 

 the physical nature of each species, a " fixed 

 direction of variation " in the sense of a phyletic 

 vital force. Instead of the SmerintkusAarvat 

 show a tendency to produce red spots on the 

 skin, it is only necessary to say these larvae tend 

 to produce red borders to the oblique stripes. 

 The latter statement would, however, be incorrect, 

 since the red borders first arose by the coalescence 

 of red spots through the action of natural selec- 

 tion. It is not even correct to say that all the 

 species of Smerinthiis show this tendency to 

 produce spots, since this character does not seem 

 to occur either in S. Quercus or S. Tremulce. 



The distinction between the two modes of con- 

 ception will become clear if we ask, as an example, 

 whether those Cheer ocampaA'd^v^ which do not at 

 present possess eye-spots will subsequently acquire 

 these markings, supposing that they maintain their 

 existence on the earth for a sufficient period ? 



The supporters of a " fixed direction of varia- 

 tion " would answer this question in the affirmative. 

 Ocelli constitute a character which occurs in nearly 



