EVOLUTION OF LATEX. 33 



Possibly owing to the suggestion of the obviously distinct sexes of 

 the tropical papaw, or melon tree, the idea of sexuality in plants is 

 widely prevalent among the aborigines of tropical America and their 

 Spanish-speaking descendants, who thus have in the word "macho" a 

 ready explanation of unproductiveness. 



Perhaps it has never occurred to any of the native rubber gatherers 

 to insist that the white man should understand the difference between 

 the ii ule macho" which is a distinct species {Costilla tunu), and the 

 il ide" termed "rnacho" because it does not yield milk, though not in 

 other respects different from the productive trees. Again has a little 

 learning proved dangerous, in that the existence of a sterile species 

 of Castilla has served as a general explanation of differing yields of 

 rubber, the true causes of which 'still remain to be discovered. 



That varietal and individual differences of 3'ield will be found inside 

 the genuine rubber-producing species is, of course, to be expected, 

 but there is also every probability that conditions, whether natural or 

 artificial, may have a profound influence on the all-important feature 

 of rubber production, so that we are brought again to our original 

 question of causes determining the formation of rubber. 



EVOLUTIONARY ARGUMENTS REGARDING LATEX. 



Some have insisted that the solution of the problem lies in discover- 

 ing the use of the rubber to the tree, on the ground that natural 

 selection brings into existence only useful characteristics. This theory 

 has encouraged speculation, and numerous attempts have been made 

 to frame a general explanation of the function of latex, or milky juice 

 in plants. Such, however, is the diversity both of the thousands of 

 latex-producing plants and of the substances which the various kinds 

 of milk contain, that any explanation sufficiently general to accommo- 

 date all might have little practical bearing on rubber culture. Indeed, 

 there is no assurance of unity of causes and methods of formation of 

 milk in the several hundred species of rubber-producing plants of 

 diverse families and conditions of growth, and we can even go farther 

 and say that Castilla itself demonstrates that the production of milk 

 and of rubber may be of no very serious importance in the plant 

 economy, since apparently normal growth and reproduction are accom- 

 plished with little or no rubber. Furthermore, we have no assurance 

 that the discovery of the function of the latex would bear directly 

 upon the question of rubber production, since it does not appear that 

 the mechanical qualities which we value in rubber, notably its elas- 

 ticity and toughness, are of use to the tree or that they exist in the 

 living latex. Commercial rubber is certainly a very different sub- 

 stance from the creamy mass which first appears when coagulation 

 sets in, and numerous changes may have taken place before even this 



4876— No. 49—03 3 



