ORIGIN OF THE LIVING MACHINE: ADAPTATION 349 



of organisms to their environment has been a matter of growth, 

 is the result of the thought of the last half-century. Previous 

 to the middle of the last century it had been assumed that or- 

 ganisms transmitted their characters so accurately to their off- 

 spring that they had continued from the beginning unchanged, 

 and that species were immutable. The immutability of species 

 (Lat. im = not + mutabilis = changing) had been assumed as 

 the foundation stone of biological science, and all conceptions 

 of nature had been based upon the idea that organisms breed 

 strictly according to their type, without change, other than 

 slight fluctuations back and forth from a center, and without 

 permanent modification. The conception which we have as- 

 sumed above that not only are all organisms constantly 

 undergoing individual variations, but that races are going 

 through a gradual series of permanent changes, resulting in the 

 appearance of new forms with successive ages was quite 

 revolutionary in thought. The belief that species were not 

 immutable, but were constantly being transformed into new 

 species by the ordinary processes of descent, changed the whole 

 aspect of our attitude toward nature. During the fifty years 

 after this conception was presented to the world for discussion, 

 it was subjected to most hostile criticism and most bitter dis- 

 pute. The objections have now, however, mainly disappeared, 

 and it has become to-day one of the accepted doctrines of science 

 that species are constantly undergoing changes, and that our 

 present species have descended from older ones and will in turn 

 develop into others. To understand and appreciate this modern 

 conception, it is necessary to survey briefly the development of 

 the idea and the fundamental facts that lie underneath it. In 

 this review we will make reference only to that phase of the 

 great theory of evolution that has to do with the origin of modern 

 species, or to organic evolution, as it is commonly termed. 



Early Views. We can trace a beginning of the idea of evo- 

 lution back to the scientists and philosophers before Christ. 

 Aristotle, nearly four centuries before Christ, recognized in a 



