SEX AND FERTILIZATION. 113 



the Baldwin Apple with pollen from the Porter, or any 

 other variety of the same species, the offspring will only 

 be a cross-bred variety, and in this kind of crossing we 

 only intermingle elements that may have been inter- 

 mingled many times before. 



True hybrids may be considered as forced productions 

 rarely found in nature, and the few that have been pro- 

 duced without the assistance of man, are but exceptions 

 to the rule. At one time, and that not many years ago, 

 hybrids among cultivated plants were so rare that it was 

 thought they must necessarily be barren, or nearly so, as 

 was supposed to be the case among hybrid animals the 

 common mule or offspring of the ass and mare, being the 

 accepted type of such animals ; but not only has the 

 mule been known to breed, but many of the hybrid 

 plants are as productive as either of their parents. Un- 

 fortunately, however, for the student of natural history, 

 it is frequently very difficult to determine species from 

 varieties among both plants and animals. Every collector 

 in any branch of natural- history, who has attempted to 

 arrange his specimens in the order laid down in " check 

 lists," or the works of the highest authorities, knows, to 

 his cost, how frequently he is compelled to re-arrange 

 his cabinet to meet the ever-changing opinions and dis- 

 coveries of those to whom he has looked as competent 

 guides in such matters. If he seeks specific information 

 that will enable him to determine what is or is not 'a 

 true species, he will find but little that is clear and defi- 

 nite on this point. Prof. Asa Gray says that the "idea" 

 of species is " based upon a succession of individuals, each, 

 deriving its existence, with all its peculiarities, from a 

 similar antecedent one, and transmitting its form and 

 other peculiarities essentially unchanged from generation 

 to generation. By species we mean absolutely the type 

 or original of each sort of plant, or animal, thus repro- 

 duced by a perennial succession of Jil^e individuals, or, 



