External and Internal Factors in Evolution \o 



j^j 



vessels." By preventing tadpoles from leaving the water, 

 Geoffroy claims that it has been shown that they can be 

 prevented from changing into frogs. The main point that 

 Geoffroy attempts to establish is no doubt fairly clear, but 

 the way in which he supposes the change to be effected is 

 not so clear, and his ideas as to the way in which new change 

 may be perpetuated in the next generation are, from our more 

 modern point of view, extremely hazy. It is perhaps not 

 altogether fair to judge his view from the standpoint of the 

 origin of adaptive structures, but rather as an attempt to 

 explain the causes that have brought about the evolution of 

 the organic world. 



During the remainder of the nineteenth century there 

 accumulated a large number of facts in relation to the action 

 of the external conditions in bringing about changes in 

 animals and plants. Much of this evidence is of impor- 

 tance in dealing with the question of the origin of organic 

 adaptation. 



The first class of facts in this connection is that of geo- 

 graphical variation in animals and plants. It will be im- 

 possible here to do more than select some of the most 

 important cases. De Varigny, in his book on " Experimental 

 Evolution," has brought together a large number of facts of 

 this kind, and from his account the following illustrations 

 have been selected. He says : " When the small brown 

 honey-bee from High Burgundy is transported into Bresse — 

 although not very distant — it soon becomes larger and 

 assumes a yellow color ; this happens even in the second 

 generation." It is also pointed out that the roots of the 

 beet, carrot, and radish are colorless in their wild natural 

 state, but when brought under cultivation they become red, 

 yellow, etc. Vilmorin has noted that the red, yellow, and 

 violet colors of carrots appear only some time after the wild 

 forms have been brought under cultivation. Moquin-Tandon 

 has seen " gentians which are blue in valleys become white 



