172 



NATURE 



\June 21, 1883 



"induction," as used in reference to the investigation of the 

 causes of phenomena. The mistake which he makes is a very 

 frequent one, and is due to the ambiguity of the words them- 

 selves, and to the inaccessibility of a treatise on modern 

 logic. 



The "deductive method," as formulated by John Mill, is one 

 method by which the mental process known as induction — " the 

 operation of discovering and proving general propositions" — is 

 accomplished. An "induction " may be asimple inference from 

 an observation ; it must be an inference in which the conclusion 

 is wider or more general than the premises from which it is 

 drawn. A " deduction" (as the term is commonly used) is a 

 result of ratiocination solely, or, in other words, of a "train of 

 reasoning," by which from a general proposition (not alone, but 

 by combining it with other proposition 1 -), we infer a proposition 

 of the same degree of generality with i'self, or a less general pro- 

 position. The " deductive method " receives its name from the 

 fact that ratiocination is combined in it with induction. 



" In order to di cover the cause of any phenomenon by the 

 deductive method, the process must consist of three parts : — (1) 

 Induction ; (2) Ratiocination ; (3) Verification ; " or in common 

 language : (1) A generalisation from observed facts [or a deduc- 

 tion from a previous generalisation] ; (2) A deduction from this 

 generalisation [or from an initial deduction] ; (3) The testing or 

 verification of the final deduction. 



The "hypothetical method " is a special and very usual form of 

 the deductive method in which in place of an induction or 

 primary deduction we have substituted a hypothesis. Under 

 proper safeguards this is the most valuable and fertile method of 

 investigating the causes of complex phenomena. Hypotheses 

 are legitimate or illegitimate. The cause suggested by the 

 hypothesis, if not already known as existing, ought to be capable 

 of being known, and, until the cause suggested is shown to 

 exist, the hypothesis, although verified, constitutes only a 

 plausible conjecture. Further, the hypothesis must be such 

 that no other hypothesis substituted for it would lead to 

 verification. 



A hypothesis, as distinguished from a proposition resulting 

 from a complete induction or a correctly formulated deduction, 

 is "a supposition without actual evidence or with evidence 

 avowedly insufficient." The whole value of a hypothesis lies in 

 the final carrying through with it of the deductive method. It 

 must be made the starting-point of deductions, and these must 

 be (one or more) brought to the test of observation or experi- 

 ment — the final process of verification. 

 So much by way of preliminary. 



The objection which my friend Mr. Thiselton Dyer has made 

 to the essay of Mr. Grant Allen upon the forms of leaves does 

 not, it appears to me, consist in a depreciation of the "deduc- 

 tive method" as Mr. William White is led to believe. Nothing 

 can be further from the real state of the case. 



What Mr. Dyer objects to is that the method is net carried 

 out by Mr. Allen. Mr. Allen gives us hypotheses — suppositions 

 with insufficient evidence — and deductions from the generalisa- 

 tion of evolution, but he is relatively deficient in " verification." 

 He also fails in the condition insisted on by Mill, who holds that 

 the hypothetical method is valueless (or relatively so) unless it be 

 proved that no other hypothesis than that formulated can be 

 similarly verified. He further, in the case of the supposed 

 exhaustion of the carbonic acid in atmospheric air, appears to 

 fail in another respect indicated by Mill, in so far as he does not 

 demonstrate the actual existence of the cause which he assumes 

 in his hypothesis. His proposition on this head is no more than 

 " a plausible conjecture " at the best, and is not a legitimate 

 conclusion arrived at by the deductive method. 



I do not think that there is any ground for discountenancing 

 either a "purely deductive" or a " purely inductive" method in 

 the treatment of biological topics, so long as the method is 

 soundly and thoroughly carried out and its logical results truly 

 and clearly stated. Still less is there any shadow of reason for 

 not fully accepting the " deductive method" (so named) as the 

 method of biological research. What we have to deprecate in 

 some modern speculative essays is the tendency to put forward 

 suppositions as though they were propo-itions which have been 

 demonstrated, and to employ the printing press in launching 

 hypotheses which are neither legitimate inductions nor deduc- 

 tions, and should be kept unpublished until their originator has 

 thoroughly examined them by the accepted " deductive method." 



E. Ray Lankester 

 II, Wellington Mansions, North Bank, N. W. 



The Peak of Teneriffe not very Active again 



With reference to my notice in Nature, vol. xxvii. p. 315, 

 stating, on the authority of a native lady in Santa Cruz, that the 

 Peak of Teneriffe was active again, even to the extent of exuding 

 a red-hot lava stream from near its summit, I am informed 

 now from a higher scientific authority, viz. a Cambridge man, 

 and high Wrangler there in his day, but since then resident in 

 Teneriffe, near Puerto Oratava, for fifty years, that that view 

 was exaggerated. I hasten, therefore, to present to your readers 

 exactly what this venerable and experienced man hrs to say, 

 without altering a word, so far as the extract goes : — 



" The facts of the case," says he, "are simply these. On a 

 clear day of south weather, about the latter end of December or 

 the beginning of January last, I happened to be looking at the 

 Peak (as I often do) and observed several distinct and very 

 copious gushes of steam issuing from the summit. In similar 

 weather 1 had often seen a similar phenomenon, but never to 

 anything like the same extent. I watched these steam gushes 

 several times that day, and very remarkable they were. On 

 going down to Port the following day, I found tbey had been 

 seen by several people there, who declared that the peak was 

 pouring forth volumes of smoke and flames. The so-called 

 smoke was simply the steam gushes I have mentioned, and what 

 were mistaken for flames I am convinced were nothing but the 

 same steam gushes illumined by the rays of the setting sun. All 

 agreed that after dark nothing was to be seen there, which cer- 

 tainly would not have been the case had there been fire or flames. 

 As for the lava stream, that was a pure fiction of an excited 

 brain. I have looked carefully at the Peak through my tele- 

 scope, and see nothing but the old, black lava streams that I 

 have known for the last fifty years, and I have spoken with one 

 of the guides who has been lately with a party to the summit, 

 and he declares he saw no trace of any eruption, or of anything 

 different from what he has always seen there." 



Then follow some other topics to the end of the letter proper; 

 but to that there is appended the following P.S., which may be 

 interesting to intending travellers this summer : — 



" Last night (May 27), about an hour and a quarter after mid- 

 night, we had a smart shock of an earthquake which woke me out 

 of a sound sleep and rather frightened us all. However, no damage 

 was done, but here people say that eruptions of the volcano are 

 always preceded by earthquakes ; so who knows but that our 

 eccentric friend's vi-ion of the three bonfires and the lava stream 

 may come to be verified after all. If the Peak has any intention 

 of erupting again, /should be personally obliged to it if it would 

 do so while I am still in the body. It would be a grand sight 

 from our Sitio." 



To any of the previously mentioned intending visitors to the 

 island I would beg to recommend that they carry Dr. Marcet's 

 recent neat little book on " Southern and Swiss Health Resorts." 

 His descriptions of Teneriffe, and especially of Guajara on the 

 great crater, and Alta Vista on the high peak, are graphic, and 

 true though terse. Indeed the only point of difference I have 

 with him is his reason for there not being forthwith erected a 

 grand hotel on the elevated Canadas, high above the summer 

 cloud level, in the driest air, strongest sunshine, and most cura- 

 tive conditions for the moist kinds of consumptive disease, which 

 the whole of this planet world has to offer. The reason he 

 gives is, that there is nothing to interest the invalids, or ordinary 

 lady and gentleman travellers up there. 



Vet there have long been mineralogy, geology, a peculiar, 

 though scanty, botany, meteorology of a most commanding 

 type, and astronomy under special advantages, inviting all the 

 readers of Nature to go there and participate in the mental 

 feast ; while now the probabilities each morning of witnessing 

 from a distance a little real eruption, will add an exciting topic 

 to the breakfast conversation and the noonday ramble. 



C. Piazzi Smyth, 

 Astronomer- Royal for Scotland 

 15, Royal Terrace, Edinburgh, June 19 



"Devil on Two Sticks" 



Why a game at once so graceful and attractive should have 

 received such a christening I do not know, and I am equally at 

 a loss to imagine how an outdoor sport like this, requiring skill 

 and promoting a healthful exercise of the muscles, should have 

 passed out of sight and become almost forgotten. Like Clerk 

 Maxwell, I have played the game many a time some twenty 

 years since, and hasten without further preliminary to describe it. 



