v.] IN THE THEORY OF NATURAL SELECTION. 265 



justified in thus assuming the existence of a new and totally 

 unknown principle until it had been proved that known 

 forces are insufficient for the explanation of the observed 

 phenomena. 



Can any one assert that this proof has been forthcoming ? It 

 has been again and again pointed out that the phyletic develop- 

 ment of the vegetable kingdom proceeds with regularity and 

 according to law, as we see in the preponderance and con- 

 stancy of so-called purely ' morphological ' characters in plants. 

 The formation of natural groups in the animal and vegetable 

 kingdoms compels us to admit that organic evolution has fre- 

 quently proceeded for longer or shorter periods along certain 

 developmental lines. But we are not on this account compelled 

 to adopt the supposition of unknown internal forces which have 

 determined such lines of development. 



Many years ago I attempted to prove ^ that the constitution 

 or physical nature of an organism must exercise a restricting 

 influence upon its capacity for variation. A given species 

 cannot change into any other species, which may be thought 

 of. A beetle could not be transformed into a vertebrate animal : 

 it could not even become a grasshopper or a butterfly ; but it 

 could change into a new species of beetle, although only at first 

 into a species of the same genus. Every new species must 

 have been directly continuous with the old one from which it 

 arose, and this fact alone implies that phyletic development 

 must necessarily follow certain lines. 



I can fully understand how it is that a botanist has more 

 inclination than a zoologist to take refuge in internal develop- 

 mental forces. The relation of form to function, the adaptation 

 of the organism to the internal and external conditions of life, 

 is less prominent in plants than in animals ; and it is even 

 true that a large amount of observation and ingenuity is often 

 necessary in order to make out any adaptation at all. The 

 temptation to accept the view that everything depends upon 

 internal directing causes is therefore all the greater. Niigeli 

 indeed looks at the subject from the opposite poiiU of view, 

 and considers that the true underlying cause of transformation 

 is in animals obscured by adaptation, but is more apparent in 



^ ' Ueber die Berechtigung der Darwin'schcn Thcoric' Leipzig, 

 1868, p. 27. 



