184 EVOLUTION AND ANIMAL LIFE 



race feeling of the confined animals seems to break down, and 

 unusual cases of hybridism are occasional!}^ noted. Also men- 

 tion must be made of the artificial induction of the fertilization 

 of sea-urchin eggs by the sperm cells of starfishes (animals not 

 only of different genera but of different classes) , and a few other 

 similar exceptional cases accomplished by Loeb and other ex- 

 perimenters. In many examples of hybridism the immediate 

 offspring are unable to produce 3'Oung and so no continuous 

 series of generations results. In other fewer cases the off- 

 spring of hybridization are fertile, and thus constitute the 

 beginnings of a new race or variety of animal or plant. Many 

 of our domesticated animal races and cultivated plant varie- 

 ties have originated by hybridism often artificially induced 

 by man. 



For the most part, however, both parents of any brood of 

 young belong to the same species, and hence they are at least as 

 like each other as the other members of the same species 

 have to be. But this may still permit great superficial dissimi- 

 larity: many attributes, such as size, color, texture, outline, 

 etc., of the body parts, especially the external ones, may be 

 quite different. For \ythin any species there may be several 

 subspecies or varieties, the individuals of all of which are 

 capable of fertile mating with each other. And even where 

 there is no distinctly recognizable subspecific distinctions there 

 may yet be much superficial dissimilarity among the individ- 

 uals composing a single species. So in all studies of the results 

 of heredity, of the actual inheritance of parental characters, the 

 degree of likeness or unlikeness of the parents must be taken 

 into account. 



So that the " laws " of heredity, as formulated on a study not 

 of its mechanism but of its results, refer to the character of the 

 parental union, whether pure or crossed, and if crossed whether 

 the parents are of different varieties of one species or of actually 

 different species. As a matter of fact the crossing of parents 

 with a few to many dissimilar characters has been the actual 

 means of getting at some of the most important evidence as to 

 the behavior of heredity that we have. For the very dissimilar- 

 ity of the parental attributes makes it possible to trace in the 

 progeny of succeeding generations the workings or results of 

 heredity with reference to these particular characters. 



Galton's Law of Ancestral Inheritance may be stated in few 



