390 



THE AMERICAN NATURALIST 



[Vol. LI 



we are perfectly within our rights when we use the term 

 ''dog" for bulldogs only. But such a statement brings 

 us no insight in the shape or the length and variability of 

 the tail in the big group of animals which everybody, ex- 

 cepting breeders of bulldogs, knows under the name of 

 ''dogs." 



We can say : ' ' Carriages have small wire rubber-banded 

 wheels" and if so we are within our rights if we limit the 

 term carriage to baby-carriages, but all such and similar 

 statements of wheat-growers, breeders of bulldogs and 

 manufacturers of baby-carriages, no matter how plausible 

 they may look to the people under consideration, have 

 this one thing in common, that they may not be general- 

 ized. Breeders of New Foundland dogs have as much 

 right to reserve the name dog for their animals, and to 

 say that dogs have long bushy tails, as the breeders of 

 bulldogs did, and if we permit the manufacturers of 

 gocarts to reserve the term "carriage" for their product, 

 and if we allow the breeders of autogamous plants to 

 limit the term species to speeies of wheat and barley and 

 peas, manufacturers of Pullman carriages certainly have 

 the right to state "Carriages are ninety-five feet long and 

 are entered by steps four feet from the ground" and the 

 breeders of sugarbeets or rye, and the zoologists will have 

 the right to state that species are variable. 



When we want to make a definition of the term 

 "species" we must make it so that it fits rat-species as 

 well as wheat-species, and in such a way that the gene- 

 ticians as well as the systematioians can apply it to the 

 things they are wont to call by the name. 



We know that all the different genes, all the different 

 inherited factors whose cooperation or non-cooperation 

 to the development of the most diverse organisms pro- 

 duces the hereditary differences among them, are each 

 in themselves invariable. We have called this invariabil- 

 ity of the genes Johannsen's law.^ Only in this way can 



3 A. L. Hagedoorn, "Wetten en Kegels in Genetica en Eugenetica," 

 Handelingen van het Genootschap van Natuur, Genees-en HeelTcunde, 1913. 



