72 ZOOLOGY 



separate wild species from one another. The more enthusi- 

 astic followers of Mendel strive to prove that the only inherit- 

 able kind of variation which they term a mutation is of 

 the kind produced from a new type of germ-cell which has 

 arisen from such a new combination of inheritable char- 

 acters as that alluded to above. Variations which consist in 

 small differences in size and proportions are termed by them 

 fluctuations. These fluctuations are due, according to them, 

 to different conditions of nourishment acting on the same type 

 of germ-cell, and such fluctuations are not inheritable. Accord- 

 ing to some of them, a natural species consists of a number of 

 different races and each race is made up of individuals pro- 

 ducing the same type of germ-cell. These races are termed 

 elementary species; they are continually intercrossing and 

 producing varied combinations, and it is, according to these 

 writers, the variety of forms thus produced that has been 

 mistaken by Darwin for continuous variability. 



Now this view, if accepted, so far from helping us to under- 

 stand the formation of natural species, brings us up in effect 

 against a blank wall. How are we to account for the differ- 

 ences between the races or elementary species ? how did they 

 originate ? To such questions the followers of Mendel have been 

 able to give no answer. It is true that a leading botanist 

 named de Vries, one of those who brought Mendel's work 

 into full recognition, has put forward the view that natural 

 species at intervals of time give rise to new "elementary 

 species." The only instance of this, however, which he has 

 been able to discover is open to grave suspicion. This is the 

 case of an evening primrose which he found growing in 

 Holland. But this plant is an introduced American form : 

 it is strongly suspected of being a hybrid, and the supposed 

 production of new varieties may be nothing more than the 

 formation of new types of germ-cell with new combinations 

 of the parental qualities such as we have just described in 

 giving an account of Mendel's doctrine. 



To sum up : the broad general considerations adduced by 

 Darwin afford an overwhelming weight of evidence that new 

 species have been and are being produced by the modification 

 of old species : and the only theory which gives any probable 

 account of how this can happen is his. Changed external 



