( Ixiv ) 
from such sources. I imagine that nobody will dispute that 
Mr. Herbert Spencer's writings have largely influenced the 
public mind — whether we agree with the details of his doc- 
trines or not— in accepting the broad principle of Evolution, 
although this profound thinker lays no claim to an expert 
knowledge of any branch of natural history. But every 
working naturalist can ascertain for himself the credentials 
of any particular writer : my remarks are simply offered with 
the object of claiming more consideration for such writers, as 
a class, on the part of practical workers. The philosophic 
faculty is quite as powerful an agent in the advancement of 
science as the gift of acquiring new knowledge by observation 
and experiment. It is not often that the faculties are com- 
bined in one individual. 
The general conclusion to which these considerations point 
is that the biological theorist, by virtue of the complexity of 
the factors, the difficulty of experimental verification, and the 
tendency on the part of the public to mistake tentative 
hypotheses for established theories, should put forward his 
views with more explicit caution than is necessary in the case 
of the physical sciences, where experimental evidence is more 
easily obtainable, and where the self- constituted philosopher 
but rarely gets a hearing. All this amounts, however, to 
nothing more than a plea for caution, and not for total absti- 
nence. To disallow speculation because a complete theory 
cannot be formed out of the existing materials, is simply to put 
a check upon legitimate advancement. I freely admit that it 
is possible to carry speculation to an unscientific extreme— to 
fritter away a plausible hypothesis by mere metaphysical 
discussion, or to bury a real and important issue under an 
incubus of verbiage. But this is not the legitimate use of 
the speculative method ; it is an accident, which the scientific 
worker will know how to avoid, and which is contingent upon 
the present condition of biological investigation. We cannot 
test our speculations offhand by a few crucial experiments, 
as in physical science, and in the meantime the logic-chopper 
may get hold of our idea and whittle it away. On these 
grounds, however, I again fail to see any reason for repressing 
speculation, It might as well be argued that because the 
