Double Adaptations 449 



chance for a sudden change in the desired di- 

 rection, as well as for the slow accumulation of 

 slight deviations. Any mutations in a wrong 

 direction would at once be destroyed, but an 

 accidental change in a useful way would be 

 preserved, and multiply itself. If in the course 

 of centuries this occurred, they woukl be near- 

 ly sure to become established, however rare at 

 the outset. Hence the positive assertion is 

 scarcely capable of direct proof. 



On' the other hand the negative assertion 

 must be granted full significance. If the al- 

 pine climate has done no more than produce a 

 transitory change, it is clear that thousands 

 of years do not, necessarily, cause constant 

 and specific alterations. This requirement is 

 one of the indispensable supports of the La- 

 marckian theory. The matter is capable of 

 disproof however, and such disproof seems to 

 be afforded by the direct evidence of the pres- 

 ent condition of the alpine varieties at large, 

 and by many other similar cases. 



Among these the observations of Holter- 

 mann on some desert-plants of Ceylon are of 

 the highest value. Moreover they touch ques- 

 tions which are of wide importance for the 

 study of the biology of American deserts. For 

 this reason I mav be allowed to introduce them 



ft'' 



here at some length. 



