32 MODERN IDEAS OF EVOLUTION 



lJ of naturalists of the development of a new species. 

 We must also admit that such is the fixity of specific 

 forms at present, and the nice equilibrium of all their 

 parts, that the changes effected under domestication 

 and by artificial selection seriously unsettle their 

 stability, and cause the varieties and races produced 

 to exist under a condition of tension and unstable 

 balance, which renders them infertile and otherwise 



( unlikely to survive if left to themselves. They have, 

 [farther, in favourable circumstances a strong tendency 



,|to revert to the original types. Again, we must admit 

 that on the supposition of slow and piecemeal altera 

 tion in a complex organism, we meet with endless 

 difficulties in relation to the origin of each change, 

 its fitting in with the other parts of the organism and 

 its maintenance while still too imperfect to be of use. 

 These difficulties are specially formidable when the 

 whole depends on favouring accidents in the absence 

 of a guiding will like that of the human breeder. We 

 also find that in the past history of life in geological 

 time, there are several great difficulties in the way of 

 the idea of slow and gradual modification. 



One arises from the fact that we can trace most of 

 ! the leading types so far back that they seem to con 

 stitute parallel rather than diverging lines, and show 

 no certain evidence of branching. The continuance 



I of the Lingulae and other Brachiopods, and of the 

 silicious sponges and the Foraminifera, from the 

 Cambrian to the modern, and more lately the history 



