4J 
gravitation must produce if really at work ; i. e., is the hypo- 
thesis consistent ? 4th, Is there any evidence of gravitation 
being actually at work in any part of the solar system, or 
other similar sphere, which would afford ground of analogy 
for regarding it as probably at work throughout the whole ; 
i.e., is the hypothesis harmonious ? Before any hypothesis 
can be admitted as certainly true, it must satisfy all these 
four requirements. Until, it does so, it can only be accounted as 
more or less credible ; provided always that it answers the 
first demand — that it is possible. If this be wanting — if 
there be no evidence that the cause assumed is a real cause, 
then the hypothesis is purely fanciful and unworthy of 
credence. But if it be possible, then so far as it fulfils the 
other three conditions it is also credible. The degree to which 
any hypothesis fulfils these conditions will depend primarily of 
course upon its truth ; it will also, however, be affected very 
seriously by the inevitable limitations of human knowledge. It 
is quite possible for a true explanation to appear inadequate or 
inconsistent, simply because of our ignorance. Thus, to take an 
illustration bearing upon the example just reviewed, the ade- 
quacy of gravitation, prior to the discovery of Neptune, appeared 
at fault, perturbations being observed in the planetary motion, 
for which gravitation failed to account. The discovery of Nep- 
tune, which removed this objection, depended, however, on a 
property altogether independent of its gravitating influence, 
the property, namely, of reflecting light. Had Neptune been 
so constituted as not to reflect (which is perfectly possible), no 
telescope could have descried it, and gravitation might very 
likely in consequence have been rejected by some as an in- 
adequate hypothesis, when, in truth, the apparent inadequacy 
arose entirely from the imperfection of our knowledge. It is 
plain, then, that no objection to an hypothesis should be re- 
garded as of final weight, for which a possible explanation can 
be given, not inconsistent with observed facts. Weaken the 
credibility of the hypothesis such objections can and do, 
destroy it altogether they cannot. On these principles, then, 
it is proposed now briefly to discuss “the credibility of 
Darwinism ." 
1. Its possibility. Are the elements involved in Mr. 
Darwin's hypothesis real elements, and are they capable of 
producing the kind of effects he ascribes to them ? The 
elements involved are four : — (1) “ Growth with reproduction ; 
(2) Inheritance which is almost implied by reproduction ; 
(3) Variability from the indirect and direct action of the 
external conditions of life, and from use and disuse ; (4) A 
ratio of increase so high as to lead to a struggle for life." 
