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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 dif- 

 ference between the "ule macho," which is a distinct species {Castilloa 

 tnmi), and the "ule" termed "macho," 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 Castilloa 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 yield will be found 

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

 pected, but there is also every probability that conditions, whether 

 natural or artificial, may have a profound influence on the all-im- 

 portant 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 is dis- 

 covering 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 suffi- 

 ciently general to accommodate 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 diverge families and con- 

 ditions of growth, and we can even go further and say that 

 Castilloa 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 

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

 the living latex. Commercial rubber is certainly a very different 

 substance from the creamy mass which first appears when coagu- 

 lation sets in, and numerous changes may have taken place before 

 even this stage is reached. Between the vegetable and animal 

 milk no complete analogy can be maintained, but it serves to il- 

 lustrate the present point if we think of the rubber not as the curd 



