DEGEl^ERATtON AND LAWS OF VARIATION. 20.'! 



opment. It is essential that the inertia of the ner- 

 vous co-ordinations be overcome, and that there be 

 made a permanent change in associations, otherwise 

 any enforced change of growth will not pass over to 

 succeeding generations. A slight disturbance in the 

 manner of growth may be produced artificially in 

 many successive generations, without producing a 

 permanent change in the hereditary impulse. Thus, 

 for many successive generations, the tails of certain 

 breeds of game dogs have been cut off ; and yet, the 

 tails of the latest generation continue to grow if not 

 disturbed. The explanation of this, and of all phe- 

 nomena of this class, seems to be, that the artificial 

 change of growth, acting late in the period of devel- 

 opment, and upon a single unimportant part, has not 

 produced so profound an impression upon the ner- 

 vous organisation as to overcome the powerful asso- 

 ciations which unite the co-ordinations governing 

 the growth of the tail with the co-ordinations gov- 

 erning the growth of the rest of the body. While 

 the tail is not important to the life of the dog, yet 

 the co-ordinations governing its growth have been 

 associated, by repetition, with the co-ordinations 

 governing the growth of the trunk since the earliest 

 appearance of the vertebrates, the tail being phylo- 

 genetically a much older organ than the legs. There- 

 fore, when there is a reaction of the co-ordinations of 

 growth of the trunk, there will also be, through 



