WHAT IS A SPECIES? 937 



specific characters, they no longer constitute a species, and cannot obviouslv 

 long continue to exist in nature. The new forms must similarly be considered 

 on their merits. If the individuals of such a form are found to occur in 

 nature, possessing definite characters distinguishing them from all other organ- 

 isms, and if they are always able to transmit such characters truly to the 

 majority of their immediate offspring, they constitute a true species, otherwise 

 they do not. It must, of course, be remembered that as a species is a group 

 of organisms, a single individual can never constitute a species, but it may form 

 the begnining or foundation of one. 



Note 6. — The fact that certain organisms can interbreed and produce fertile 

 offspring does not necessitate the combination of such organisms in one species 

 or vice versa. This is shown by the fact that illegitimate unions between the 

 different forms of flowers occurring in one and the same undoubted species, 

 in dimorphic and trimorphic plants, produce very little fertile seed, while 

 the plants raised from such seed are sterile, just as is so frequently the case 

 in illegitimate unions between forms belonging to different species. 



Note?. — Attention has been paid during recent years to the cultivation of 

 plants and the breeding of insects for long periods with the object of observ- 

 ing the kind and amount of variation which occurs among the offspring of one 

 and the^same individual, and of obtaining an idea of the process of evolution 

 which has produced the forms now existing in nature. If, during such oper- 

 ations, among the offspring of an individual belonging to a natural species 

 (A) a form is sooner or later found to occur which appears to exactly resem- 

 ble another form which occurs wild in nature, and which has been classed as a 

 distinct species (B), this in itself is no proof that (B) has directly descended 

 from (A) in nature, for the organisms kept under observation have developed 

 under artificial conditions, and it is conceivable that, in nature, (B) has arisen 

 from a distinct species (C), under the influence of different conditions of exist- 

 ence through a different series of variations. Moreover, even if proof has been 

 obtained that a certain group of individuals has descended from another 

 group, this alone would not be a sufficient reason for combining them as one 

 species, for, in the progress of time, the intermediate forms which once united 

 the two groups may have disappeared, causing them to occur in nature as 

 distinct species, separated by well-marked and constant differences. 



Note 8. — For the correct classification of forms as species, sub-species, races 

 or varieties a knowledge of the life-history, as well as of the appearance 

 of the seasonal forms of individual organisms, and of the kind and amount of 

 variation which may occur among the immediate offspring of one and the same 

 individual, in nature, is essential. 



The systematist, however, must rely for his determinations mainly on mor- 

 phological characters, coupled with his knowledge of the development and varia- 

 tion of the few forms which have been studied. His conclusions therefore 

 must be liable to error. If the present, or similar, principles are accepted 

 however it will be possible to obtain proof whether, or not, his conclusions are 



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