268 BIOLOGY 



common flowers both male and female organs are developed 

 in each flower, but there is almost always some means which 

 prevents the flower from self-fertilization and insures cross 

 fertilization. In a few animals and plants, it is true, self-fertil- 

 ization appears to be the rule, but it is very unusual. 



It appears that the reason why cross fertilization is so com- 

 monly found, is that it results in more or stronger offspring. 

 Experiments carefully carried out in plants have shown that, 

 in many cases at least, the offspring resulting from cross fertil- 

 ization are more vigorous than those coming from close fertil- 

 ization. In animals there is less evidence at hand on the 

 subject, but here, too, it has been recently shown that, in some 

 cases at all events, cross fertilization is more productive of a 

 vigorous progeny. Apparently, then, cross fertilization is based 

 upon a fundamental law. 



Hybrids. On the other hand, it is necessary that the sperm 

 that unites with the egg shall come from another individual 

 not too unlike the one that produces the egg. If the egg be- 

 longs to one species of animal or plant and the sperm to another 

 species, they are not likely to unite at all. If two different spe- 

 cies are crossed the rule is that there is -no offspring, or that, 

 if there is offspring, they will themselves be incapable of pro- 

 ducing young. Such an individual is known as a hybrid, 

 and frequently hybrids are sterile. It was at one time sup- 

 posed that they were always sterile, a conclusion that was 

 based largely upon the fact that the mule, which is a hybrid 

 between a horse and an ass, is well known to be incapable of 

 breeding. But most careful study of both animals and plants 

 has shown many instances where hybrids are fertile, so that 

 the sterility of hybrids is by no means a fixed rule. In general, 

 however, in order to produce the most vigorous offspring it is 

 necessary that the eggs of one individual should be fertilized 

 by sperms from another individual of the same species, but 

 not too closely related. Close inbreeding has a tendency to 

 foster weakness. 



