BIOLOGY IN OUR ARTS CURRICULUM. 99 
pose, with the late G. H. Lewes and Dr. Roux, that there is an 
intercellular struggle going on in the tissues, and consequently 
an intercellular selection that causes variations in the organs, is 
merely to shift the difficulty one step backward, and by no 
means to explain the cause of the variations. To say, as some 
do, that variations arise by chance, is only another way of saying 
we cannot explain them. To say that no two individuals, or no 
two cells, are exposed to exactly the same conditions, and that, 
therefore, variations must arise—or, in other words, that all 
variations are caused directly by external conditions—is to draw 
an inference for which there is no proof, and one that is appa- 
rently contradicted by a large array of facts of different kinds. 
No doubt external agencies have some direct effect in producing 
individual variation, but it is doubtful whether these variations 
are ever transmitted, and in almost every case it is impossible to 
feel sure that a variation supposed to be due to direct external 
action is not in reality due to selection. It would be more than 
rash, it would be foolish, to assert that we can never know more 
than we do now on the subject, but we may safely say that at 
present we know next to nothing. When we try to investigate 
the cause, or causes, of variability, we are confronted with 
nothing but difficulties, and we have no firm ground from whence 
to meet them. But starting from the ascertained fact, that 
numberless variations are constantly arising, we can explain by 
the principle of selection how these variations accumulate in 
different directions, until in time the most diverse organisms are 
produced. 
It is well known that in countries like Europe, long settled 
by civilised man, the total number of individual animals remains 
about the same, although many are born each year. Evidently 
this is due to the fact that the district is only capable of fur- » 
nishing food for a certain number, and when that number is 
reached no further increase can take place. It follows frcm 
this, that in each year as many individuals must die as are born; 
and as most animals and plants produce annually more than 
two young, it also follows that during each year the number of 
individuals that must perish is greater than the number that can 
survive. As therefore the supply of food is limited, there must 
always be, between all animals and plants, a severe competition 
for existence, a struggle to survive at the expense of neighbours. 
The individuals that die will usually be those that are least 
adapted for living—either for procuring food, or for escaping 
enemies. Useful variations will tend to increase, and succeeding 
generations will diverge more and more from the original stock. 
This is due to what is called natural selection. 
But there is another mode of selection which also causes each 
succeeding generation to diverge more and more from the 
original stock. This is best seen in the artificial selection by 
man of the plants and animals he has domesticated. Artificial 
selection may be either methodical, as when a man endeavours, 
by breeding from chosen individuals, to modify a breed 
