EVOLUTION AS SEEN IN THE CLASSIFICATION OF ANIMALS 319 



myscus inhabiting the Dismal Swamp ) . As a result, jorms intermediate in 

 structure or transitional between the two species are not usually found. The 

 body structure of one species is usually separated from the structure of 

 neighboring species by a '^bridgeless gap." The word "usually" is important 

 in this statement, since many exceptions occur. 



We may summarize our discussion of the species as a unit of classification 

 and evolution by listing the following attributes which in general character- 

 ize species: (1 ) members of a species possess in common distinctive char- 

 acteristics; (2) intermediate or transitional forms are not usually found; 

 (3) members of separate species do not usually interbreed, or if hybrids 

 are produced they are usually sterile; (4) allied species usually have sepa- 

 rate, though frequently adjoining, territories. 



It is safe to surmise that any population of animals having all four of the 

 attributes just fisted will be considered a distinct species by all biologists. 

 Disagreement enters when a group has some of the attributes but not afi. 



Finally, we may mention the practical difficulty of applying the yardstick 

 of reproductive isolation to populations which are separated geographically 

 (are allopatric). // such populations came into contact in nature would 

 they interbreed? One method of answering the question is to attempt arti- 

 ficial hybridization between them. But here the evidence is one-sided. If 

 the two populations will not interbreed in the laboratory (assuming that 

 each population will breed within itself under such circumstances), that fact 

 would be considered evidence that the populations are in fact separate spe- 

 cies. Suppose, however, that the populations do interbreed in the laboratory: 

 that fact is not in itself evidence that the populations should be considered 

 to belong to one and the same species. For in a state of nature they might 

 not interbreed, and if they did not they would be as reproductively isolated 

 as though they could not. For this reason, and because many animals will 

 not breed in captivity anyway, the experimental approach to the question 

 has limited usefulness. Hence frequently the question can not be answered 

 directly. Accordingly, systematists attempt to solve the dilemma by deciding 

 whether or not the amount of morphological difference between the two 

 populations is great enough so that they probably would not interbreed // 

 they did come into contact naturally. But this brings us back to the variable 

 of differences in judgment between individual biologists as to how much 

 morphological difference is necessary if populations are to be regarded as 

 separate species. Truly there is no royal road to classification. Nature is 

 enormously complex. The complexity of the problems presented affords 

 much of the challenge and fascination in science. If all the problems were 



