Reviews 
130 
of different families. In such cases when crossing occurs some of the species 
may disappear and others originate. 
Species formation is also possible in allogamous organisms inhabiting a 
large area where barriers are absent but the rate of dispersal is very slow 
(e.g. snails in a lake), for there the conditions in every spot will approach 
those of complete isolation, owing to the sedentariness of the local populations, 
so that a great number of local forms, the despair of the systematist, may come 
to exist. Very close local study will certainly distinguish a number of these 
local forms, while the “lumper” will include the whole series in one species. 
Organisms with greater powers of dispersal are much more likely to be mono- 
typic over a wide area (e.g. trout in the same lake). 
These considerations lead on to the question as to whether, in the light 
of modern genetic research, we can obtain a logical and workable definition 
of the term species. The authors define it as a group of organisms so constituted 
and situated that they tend, under conditions which promise to be permanent, 
to reduce automatically their potential variability, and this is claimed as corre¬ 
sponding well with the conception of the taxonomist. The usual procedure of 
the taxonomist, the description of a “type specimen,” is certainly founded on 
the common experience that an enormous majority of the plants or animals 
grouped under the name conform to the description. The prevailing opinion 
among systematists is that species are realities, real groups of organisms, of 
which the majority are true to type, and it is unlikely that this opinion is 
so devoid of foundation as some modern geneticists would have us believe. If 
the authors are correct in their main contentions we can see that it must 
correspond with reality; and that it is due to the fact that in spite of crossing 
there is always operative in greater or less degree an automatic reduction of 
the variability of each group of individuals which we call a species. A variety, 
on the other hand, differs sufficiently from the specific type (owing to chance 
combination of gametes both lacking a certain gene, or supplementing one 
another’s genotype and thus producing a new character) to demand a name, 
but has no permanence—it does not commonly reproduce itself as such for 
many generations, but is reabsorbed into the species. A variety may become 
a species if it is isolated. Thus in autogamous plants there will be no varieties. 
For instance Jordan’s “elementary species” of Erophila vulgaris (Draba verna) 
are real species. 
According to Darwin there is no fundamental distinction between varieties 
and species—varieties were for him incipient species. This is only true, according 
to the authors, if the condition of isolation supervenes which it generally does 
not. Darwin clearly showed that there is no fundamental difference between 
the points which distinguish species in nature and those which separate breeds 
of domestic animals or strains of cultivated plants. In this last contention, 
according to the authors, he was perfectly right. But, they add, domestic 
breeds are species. They fulfil the conditions of the definition in every respect. 
In their final chapter the authors consider the status of man, and the races 
and nations of mankind in relation to their general view of evolution and 
species formation, and they are forced to conclude that the factors which keep 
populations or groups within a population apart have the same effect as among 
other animals. The logical deduction would seem to be that not only do the 
different nations contain separate species of man, but that all groups within a 
