I0 8 VARIATION 



of a rat and grafted it back into its own body. After it had 

 united he severed it at the normal base and thus provided the 

 animal with a " reversed tail." He found, however, that the 

 tail of the mouse did not grow as well in the body of the rat and 

 would not unite at all with the body of either the dog or the cat. 1 



Born succeeded in uniting the anterior and posterior parts of 

 the tadpoles of two different genera of frogs (Rana esculenta 

 for anterior and Bombinator igneus for posterior). The combi- 

 nation lived for ten days, when it was killed because of patho- 

 logical changes. 2 



In the same way Harrison made up an individual of two 

 species (Rana virescens and Rana palustris). This he kept alive 

 until after its transformation into a frog, " each half retaining 

 the characteristic features of the species to which it belongs." 2 

 This being true, it is not surprising that many varieties of 

 apple can be grafted into the same tree top. Examples of this 

 sort might be multiplied indefinitely, as in the making up of 

 worms by grafting together pieces of two different species, in 

 which each piece preserves its specific characters ; but enough 

 has been given to show the persistence with which specialized 

 tissue continues to discharge its natural function even under the 

 hardest of conditions. 3 



The circumstances under which living matter can discharge 

 its normal functions unaltered, either in character or in degree, 

 and the limits beyond which these functions must cease or 

 undergo alteration, all this is not only a question of deep 

 biological interest but it is one of special significance in breed- 

 ing, because it throws no little light upon the real nature and 

 causes of variability, a subject upon which we sorely need in- 

 formation if variations are ever to be controlled either in their 

 development or in their transmission. 



Summary. We are to regard variations in function as well 

 as in form, of activities as well as of structure, of what an 

 animal or plant does, as well as what it is. 



The body functions are not constant, but variable. They are 

 variable as between different individuals and also with the same 



1 Morgan, Regeneration, pp. 178-179. 2 ibid. pp. 183-185. 



8 Ibid. chap, ix, pp. 159-189, " Grafting and Regeneration." 



