THE PARALLEL GROWTH OF OPPOSITES 25 



for existence, it is not from his more highly developed 

 competitors that man has cause to fear. The primitive 

 forms of microscopic life are still his most vigorous and 

 most formidable assailants. 



Not only is it true that the same causes which produce 

 increased adaptation, at the same time produce misadapta- 

 tion, but the general process of change does not necessarily 

 imply an increase of either. The definition of evolution 

 as growth, accompanied by increase of structure, applies 

 only to a limited proportion of the whole number of results, 

 whose explanation must be sought for in the varied inter 

 action of the same general laws. 1 Besides what may be 

 called the forward type of evolution, we find some cases 

 where the process is one of simplification of structure, and 

 others where the change is merely qualitative, and does not, 

 as far as observation tells us, affect the complexity of the 

 organism in either direction. The barnacle is a favourite 

 example of the first, and the caterpillar, which exchanged 

 a brown colouring pigment for a green, of the second type 

 of change. In the matter of mere growth also, as well as 

 of structure, the progress may be in either direction. The 

 male of the spider, the dwarf elephants whose fossil remains 

 have been discovered in Malta, the pygmies of Central 

 Africa, are examples of a loss of size which is not retro 

 gressive, as it represents no previous stage in the generic 

 history. How great a share of the whole process is supplied 

 by the action of what may be called degenerative evolution 

 we do not yet know, but it is at any rate far too con 

 siderable to be overlooked in calculating the future from 

 the past. 



Neither do we clearly understand under what conditions 

 forward evolution is arrested, and the tendency to increased 

 complexity gives place to a process of simplification ; but 

 it shows how little attention has been paid to this most im 

 portant aspect of biological history that the increased 

 plenty and security, which are reckoned by the synthetic 

 philosopher among the distinctive features in advancing 

 1 H. Spencer, Biology, i. 133, note 



