INTERPRETATION OF NATURE. 43 
What I mean is that, having grown a long neck in the forests, 
the giraffe would then be able to wander off and to feed in places 
barren of undergrowth, and, indeed, might take unrivalled 
possession of the district. So, too, is it not possible that our 
xerophytic plants which grow in dry (or very boggy) places 
do so, not by reason of a vigorous eradication of the thirsty 
ones, with the consequent survival of those whose wants were 
small, but because requiring less they were able to migrate 
to situations where competition was not so rife ? 
In a word, does environment always select individuals, 
and do not individuals sometimes select their environment ? 
Does every persistent variation necessarily have a selected 
value, or, indeed,a use? Istherea reason why any hereditary 
variation (saving a detrimental one) should not persist ? 
Once we admit the probability of such persistence, we must 
greatly modify our conception of the genesis of species, as, no 
doubt, many have already done. 
But this preconception, of which we are all guilty, warps 
our observation in another way. It often blinds us to excep- 
tions, and the progress of science depends nowadays, not so 
much on the propounding of great laws, as on the discovery of 
exceptions to the rule. One tends, in the light of some 
absorbing theory, to pass exceptions by, or to put off their 
discussion till they can be made to fall in line with our pet 
ideas. Indeed, we are often so bent on the discovery of facts, 
which substantiate our views, that the small exceptions pass 
unobserved by us, and need to be repeated many times before 
their significance can be realized. In this connection let me pass 
on to you Professor Bateson’s words of counsel: ‘ Treasure 
your exceptions ; when there are none the work gets so dull 
that no one eares to carry it further; keep them always 
uncovered and in sight. Exceptions are like the rough brick- 
work of a growing building, which tells that there is more to 
come, and shows where the next construction is to be.” 
You have only to look back on the history of any branch of 
science to see how valuable exceptions have been, and if you 
think of it, perhaps, you will agree that the rule which has most 
exceptions is, on the whole, likely to be the truest, and nearly 
always has references to the most facts. The fewer the facts 
