270 Sir John Lubbock’s Address. 
Darwin commenced his work by discussing the causes and 
extent of variability in animals, and the origin of domestic 
varieties ; he showed the impossibility of distinguishing be- 
tween varieties and species, and pointed out the wide differences 
which man has produced in some cases—as, for instance, in our 
domestic pigeons, all unquestionably descended from a com- 
mon stock. He dwelt on the struggle for existence (which has 
since become a household word), and which, inevitably result- 
ing in the survival of the fittest, tends gradually to adapt any 
race of animals to the conditions in which it occurs. 
While thus, however, showing the great importance of natu- 
ral selection, he attributed to it no exclusive influence, but 
fully admitted that other causes—the use and disuse of organs, 
sexual selection, etc.—had to be taken into consideration. 
Passing on to the diificulties of his theory he accounted for the 
absence of intermediate varieties between species, to a great 
extent, by the imperfection of the geological record. Here, 
however, I must observe that, as I have elsewhere remarked, 
those who rely on the absence of links between different species 
really argue in a vicious circle, because wherever such links co 
exist they regard the whole chain as a single species. The 
dog and jackal, for instance, are now regarded as two species, 
but if a series of links were discovered between them they 
would be united into one. Hence in this sense there never 
can be links between any two species, because as soon as the 
large genera, as, for instance, Rubus and Hieracium, with refer 
ence to the species of which no two botanists are agreed. __ 
The principles of classification point also in the same diree- 
