40 
variation tending the other way leads to extinction and so 
at last, hypothesis after hypothesis dying out, that one s is left 
alone as victor which is found to he most perfectly adapted to 
the exigencies of the case. . . » 
I have been led to these remarks upon the relation of hypo- 
theses to scientific conclusions, 1st, because it is such an 
intermediate hypothesis which we have this 
sider; and, 2nd, because it is sometimes said that to spend 
time in estimating the credibility of an unproved theory is 
unscientific, we should rather devote ourselves to the investi- 
gation of facts. But if the principles laid down above be 
correct, and it is practically impossible to conduct investiga- 
tion without hypotheses, then it is clearly a matter of grave 
importance what hypotheses we thus tentatively adopt,- a 
matter which should of course be determined by the amount 
of credibility at present belonging to them. Some facts, 
some arguments, some analogies, bearing upon Darwinism we 
have already, albeit confessedly insufficient to demonstrate 
its truth. The question is, then, do these facts, arguments 
and analogies afford such an amount of evidence in its favour 
as to render it a fair working hypothesis for futuie leseaic . 
Is Darwinism, in a word, credible? Not, is it the true ex- 
planation of the phenomena it seeks to account for; but, 
it such an hypothesis as may possibly m the end prove to be 
so ? Jf go— if Darwinism be credible, then it ought to be 
carefully kept in mind, applied, and tested, in all inves le- 
gations into the facts which it concerns, that so its tiuth 01 
falsity may become apparent. If, on the other hand, Darwin- 
ism be incredible, then it may be at once rejected as unneces- 
sary to be considered, 
its favour is adduced. 
at all events until fresh evidence in 
This is the practical issue which the 
present paper proposes to raise. . . ,, 
F The tests to which scientific hypotheses are subject m t e 
process of selection by which they pass into certain interpre- 
tations, are fourfold-possibility, adequacy consistency and 
harmony. The precise meaning to be attached to these terms, 
and the value to be set upon the tests they denote, may be 
best seen by a simple example. Let us take for this purp 
the hypothesis that gravitation is the sole controlling force by 
which the motions of the planets in the solar . system are 
regulated. To test the credibility of this hypothesis we should 
have to inquire, — 1st, Is gravitation a real cause, capable y 
its action of controlling planetary motion ; %. e., is the hypo- 
thesis possible ? 2nd, Is gravitation a sufficient . cause to accou 
for all the motions actually observed ; %. e., is the 
adequate? 3rd, Are all the effects m fact produced which 
