IN THE THEORY OF NATUKAL SELECTION. 259 



known internal forces which have determined such lines of de- 

 velopment. 



Many years ago I attempted to prove x that the constitution or 

 physical nature of an organism must exercise a restricting in- 

 fluence 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 con- 

 tinuous with the old one from which it arose, and this fact alone 

 implies that phyletic development must necessarily follow certain 

 lines. 



1 can fully understand how it is that a botanist has more incli- 

 nation than a zoologist to take refuge in internal developmental 

 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. Nageli indeed looks at the 

 subject from the opposite point of view, and considers that the 

 true underlying cause of transformation is in animals obscured by 

 adaptation, but is more apparent in plants 2 . Sufficient justification 

 for this opinion cannot, however, be furnished by the fact that in 

 plants many characters have not been as yet explained by adapta- 

 tion. We should do well to remember the extent to which the 

 number of so-called ' morphological ' characters in plants has 

 been lessened during the last twenty years. What a flood of 

 light was thrown upon the forms and colours of flowers, so often 

 curious and apparently arbitrary, when Sprengel's long-neglected 

 discovery was extended and duly appreciated as the result of Dar- 

 win's investigations, and when the subject was further advanced 

 by Hermann Muller's admirable researches ! Even the venation 

 of leaves, which was formerly considered to be entirely without 

 significance, has been shown to possess a high biological value 



' Ueber die Berechtigung der Darwin' schen Theorie.' Leipzig, 1868, p. 27. 



2 1. c., Preface, p. vi. 



S 2 



