WHA T IS E VOL UTION? 23 



to be itself a cause, which is in the nature of things 

 impossible. The causes of development must always 

 be distinguished from the evolution itself. It has 

 been the fashion to use the expression factors of 

 evolution to cover the causes ; but it would be more 

 honest to admit at once that there must be efficient 

 and adequate causes for every development. 



3. The term Evolution is used to express in 

 differently all changes of the nature of development, 

 however different in kind from each other. Spencer s 

 definition that evolution is the transformation of the ^ : 

 homogeneous through successive differentiations into^ 

 the heterogeneous would cover creation as well as 

 development, in the sense in which he understands it, 

 and it does not cover those developments in which 

 the complex becomes more simple, as in what is 

 termed retrograde development in plants and animals. 

 But this definition covers, as used by Spencer and 

 Darwin, even with reference to organisms alone, three 

 distinct things : (i) Direct development of structures 

 previously prepared and subjected to the action of 

 adequate causes, as heat, moisture, air, &c. Of this 

 kind is the development of seeds and eggs into perfect 

 plants and animals. This is the only kind which can 

 be termed spontaneous, and this term can be applied 

 only in a very limited sense, because it implies a 

 previous laying up, potentially or structurally, in the 

 germ, of all that is to be developed from it. (2) In 

 direct development, or that which takes place under 



