430 KINSHIP AND ADAPTATION 
proved to be satisfactory to the best thinkers of many ages, 
including many eminent naturalists, 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 offspring 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 different 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 unmistakably 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 perfectly fertile 
offspring may be used as a final test. Species thus viewed 
