PHYSICAL AND BIOLOGICAL THEORIES 51 



behaviour of gases under pressure with Darwin's theory of natural 

 selection. In the former case much thought was needed to gather 

 the data ; but they could be exactly measured, and, once obtained, 

 a single induction sufficed to enable the discoverer to perceive 

 his law ; and, though a great deal of testing was still necessary, yet 

 the means of performing it were not hard to think of. In the 

 latter case, the facts lay patent but multitudinous under the eyes 

 of every man. Individuals, as regards some of their characters at 

 any rate, could be measured with exactness ; but the measurements 

 of no child agreed with those of its parents. The difficulty lay, 

 therefore, not so much in observing facts as in interpreting them 

 in tracing hidden analogies, in seeing likenesses amid differences, 

 in separating the relevant from the irrelevant, essentials from non- 

 essentials, in brushing aside the veil of familiarity, in discarding 

 superstitions, in devising tests. Boyle's difficulties, however 

 great, were not of the same order as Darwin's. They lay 

 mainly in discovering the facts, all of which were obscured. 

 Darwin's difficulties lay in tracking his way through the enormous 

 and confusing maze of patent facts that spread and grew around 

 him. 



8 1. Obviously biology does not lend itself to experiment so 

 readily as physics and chemistry. Apart from the circumstance 

 that many of its facts are patent, it is too complex. The first 

 interpretation of a physical or chemical experiment is often 

 accepted without cavil, and is comparatively rarely overthrown. 

 But seldom is the first interpretation of a biological experiment 

 accepted without hot controversy, and it is quite usual for the 

 original interpretation to be overthrown and replaced by another, 

 and another, and yet another. These difficulties of interpre- 

 tation indicate the difficulties of satisfying the conditions 

 which make experiment useful the difficulty of so eliminating 

 irrelevant complications, that in the case of each experiment 

 only one interpretation of the result is possible, or at least 

 reasonable. 



82. The notion that deduction is necessarily illegitimate and 

 dangerous is demonstrably incorrect. That opinion has never guided 

 successful students in their efforts to interpret nature. It is illegiti- 

 mate only when we start with an unproved assumption, or when it is 

 used merely to expand untested inductions by thinking which cannot 

 itself be tested by an appeal to reality. Rightly used, it is our 

 prime means of securing accuracy when thinking about sequences of 

 events. Physicists have constantly used it, and with magnificent 



