430 KINSHIP AND ADAPTATION 



proved to be satisfactory to the best thinkers of many ages, 

 including many eminent naturahsts, it must have afforded a 

 reasonable explanation of numerous facts, and we may be 

 sure therefore that it contains important truth. 



A species does seem to be fixed in the sense of having nat- 

 ural limits beyond which unlikeness among its members 

 cannot go. Thus even breeders of domesticated varieties 

 find that they cannot induce more than a certain amount 

 of modification in any one direction. For example, careful 

 experiment has shown that if seeds from a wild carrot be 

 planted in rich soil, and then seeds from the offspring with 

 largest root be similarly planted and tended, and the same 

 process of selection and planting be continued for several 

 generations, there will finally be obtained as large rooted 

 plants as any in cultivation. But sooner or later a size is 

 reached beyond which the root does not increase; and if the 

 most highly cultivated carrot-plants scatter their seed over 

 neglected ground, as too often happens, the plants which are 

 thus allowed to "run wild," as the saying is, soon become 

 indistinguishable from the wild carrots which are pernicious 

 weeds. 



Another common experience of breeders is their inability 

 to obtain fertile oft'spring by mating individuals of different 

 species. It is true that pollen from a white oak may cause 

 the ovules of a post-oak to develop into seeds which may grow 

 into trees perceptibly chfferent from either parent; and such 

 hybrids are occasionally met with in nature. But when 

 carefully observed it is usually found to be true either that 

 hybrids are incapable of bearing offspring, or that such off- 

 spring as they have are apt to belong umnistakal:)ly to one or 

 the other of the parent species. Many of the so-called hybrids 

 of horticulturists are merely crosses between varieties of the 

 same species and their fertility does not affect the above rule. 

 Here, then, seems to be another definite limit circumscribing 

 a species as if some law of fixity had been imposed upon it 

 from the beginning. Many naturalists have maintained 

 that in case of doubt as to whether two forms are true species 

 or merely varieties, the power to produce jierfectly fertile 

 offspring may be used as a final test. Species thus viewed 



