ARE SPECIES STABLE? 37 



Among plants, at all events, there are many in- 

 stances of hybrids between species being perfectly 

 fertile, and continuing so for an indefinite period. 

 Experiments during the last twenty-five years have 

 increased the number of such fertile crosses many 

 fold. Direct experiments upon plants are very 

 easily carried on, for the pollen can readily be trans- 

 fered from one plant to another. By these experi- 

 ments, so great an amount of crossing has been 

 found possible, that one writer comes to the conclu- 

 sion that fertility of hybrid plants is the rule and 

 sterility the exception. So far as plants are con- 

 cerned, there is not the slightest ground for consid- 

 ering sterility as a distinctive bar, separating species. 

 Animals are not so easy to study, and the diffi- 

 culties in the way of experimenting have prevented 

 any extended observations. Of course, only those 

 animals under confinement can be studied, and con- 

 finement always produces great changes in the repro- 

 ductive function. Some animals will not breed at all 

 when in a state of captivity or domestication, while 

 others appear to breed more freely than in their na- 

 tural condition. It is difficult to get two species to 

 unite so as to make it possible to discover whether or 

 not they are fertile. But nevertheless quite a num- 

 ber of cases of cross-breeding are known. Two 

 species of apes, belonging to different genera, have 

 crossed and produced young ; the rabbit and the 

 hare very commonly cross ; the tiger has bred with 

 the lion ; the leopard with the jaguar ; the polar 

 bear with the brown bear ; various species of the 

 horse family have repeatedly crossed. A hybrid 



