82 ANIMAL CLASSIFICATION— TAXONOMY 



cies which causes the reproductive cells to be incompatible. It might be 

 mentioned at this point that there is a popular belief that a cat and a rabbit 

 can be bred together to give an animal which is known as a "cabbit," but 

 such a belief is false. This lack of sexual attraction and incompatibility of 

 reproductive cells are sufficient to keep most species pure, but there are a 

 few very closely related species of animals which do mate and produce 

 offspring. A donkey and a horse are different species, but they will 

 breed together and produce a mule ; and a cow and an American buffalo 

 may be bred to produce an offspring known as a cattalo. Both the mule 

 and the cattalo are sterile, however, and the mixed species cannot con- 

 tinue beyond this one generation. This sterility is due to the differences 

 in the chromosomes of the two species which have been crossed. In cer- 

 tain rare cases there may be offspring from the first generation hybrids 

 — there are a few authentic cases of mules bearing colts for instance — 

 but these are too uncommon to have great practical significance. 



With these facts in mind we could say that different species either do 

 not mate with one another or, if they do mate, they produce no offspring or 

 offspring that are more or less sterile. Were it not for this barrier to cross- 

 ing, think what a conglomeration of organisms we would have on the earth. 

 Each individual would be a type unto itself — there would be no clear distinc- 

 tion between cats, dogs, and bears, or horses, cows, and sheep. Instead, 

 there would be every gradation imaginable between animals that had no 

 geographical barriers or other barriers to prevent crossbreeding. 



This distinction seems to make it very clear that a species is a biological 

 entity, and man needs only to discover the lines of division and give names 

 to the species. There are many cases, however, where such a criterion can- 

 not be applied. Many primitive forms of plants and animals do not have 

 sexual reproduction ; and, in these, the distinctions between species must 

 depend entirely upon the judgment of workers based upon morphological 

 and physiological features, since there can be no crossbreeding. Also, many 

 species that have sexual reproduction have been named on the basis of 

 morphological and physiological features because it has not been practical 

 or convenient to observe the results of crossbreeding. Hence, we cannot 

 cite any clear-cut universal criterion for the distinction of species which 

 will apply in all cases— it is frequently human convention rather than bio- 

 logical entity that determines the distinction. 



REVIEW QUESTIONS 



1. Why is taxonomy a necessary part of biological science? 



2. Who established the modern method of taxonomy? Describe his work. 



3. What are the advantages of scientific names over common names? 



