164 BIOLOGICAL LECTURES. 



The ancients looked upon the whole animate world as having 

 been directly created by some higher power, and believed that 

 each species indicated a distinct and individual creation. They 

 also observed these differences between the plants of different 

 regions, and, in fact, between those of the same region. They 

 thought that all living organisms were created to fit the 

 special location and conditions under which they existed. 

 They believed that water animals and water plants, for 

 example, had always been and would always remain such ; 

 that the desert region had always been a desert region, and 

 that the Cactus had been created to fit it. We are not 

 obliged to go very far back in the history of our own times 

 to find eminent naturalists who have strongly advocated this 

 view. No less a man than Louis Agassiz was one of the 

 most strenuous defenders of this theory. He believed that 

 species of both plants and animals were invariable, living 

 and dying in form and color and habit just as they were 

 created. And there were many other naturalists the world 

 over who held to this doctrine of special creation. It must 

 not be forgotten, however, that though their views attained 

 no prominence, such men as Lamarck had at different times 

 indicated a strong belief in the variability of forms. 



During the last twenty years there has been a great revision 

 of thought in regard to these subjects. At present all 

 naturalists believe not only in the possibility of a continual 

 change in both plants and animals, to fit them for varying 

 conditions, but also in the gradual growth and development of 

 very diverse changes in climates, rendering equal modifications 

 in both fauna and flora absolutely essential to their continued 

 existence. Since great changes have always been slowly taking 

 place in the earth's surface and climate, and since present 

 vegetation has always been subject to these gradually varying 

 conditions, it must have adapted itself, with a slowness commen- 

 surate with the earth's changes, to the ever-appearing new 

 conditions. In this way, we think, can be solved the problem 

 of the infinite variety of vegetation. 



By experiment we may determine the two factors underlying 

 plant variation. On the one hand the laws of inheritance are 



