TRANSACTIONS OF SECTION D. 691 



descend in tlie scale, external morpLology becomes more and more insecure as a 

 guide, and a thorough knowledge of the minute structure and life history of each 

 organism becomes indispensable to anything like a correct determination of its 

 taxonomic position. The marvellous theory of the true nature of Lichens would 

 never have been ascertained by the ordinary methods of examination which were 

 held to be sufficient by lichenologists. 



The final form of every natural classification — for I have no doubt that the 

 general principles I have laid down are equally true in the field of zoology — must 

 be to approximate to the order of descent. For the Theory of Descent became an 

 irresistible induction as soon as the idea of a natural classification had been firmly 

 grasped. 



In regard to flowering plants we owe, as I have said, the first step in a natural 

 classification to our own great naturalist John Ray, who divided them into Mono- 

 cotyledons and Dicotyledons. The celebrated classification of Linnteus was 

 avowedly purely artificial. It was a temporary expedient, the provisional character 

 of which no one realised more thoroughly than himself. He, in fact, himself gave 

 us one of the earliest outlines of a truly natural system. Such a system is based on 

 affinity, and we know of no other explanation of affinity than that which is implied 

 in the word, namely, common parentage. No one finds any difficulty in admitting 

 that, where a number of individual organisms closely resemble one another, they 

 must have been derived from the same stock. I allow that, in cases where external 

 form is widely different, the conclusion to one who is not a naturalist is by no means 

 so obvious. But in such cases it rests on the profound and constant resemblance 

 of internal points of structure. Anyone who studies the matter with a perfectly 

 open mind finds it impossible to draw a line. If genetic relationship or heredity 

 is admitted to be the explanation of affinity in the most obvious case, the stages 

 are imperceptible, when the evidence is fairly examined, by which the same con- 

 clusion is inevitable, even in cases where at the first glance it seems least likely. 



This leads me to touch on the great theory which we owe to Mr. Darwin, 

 That theory, I need hardly say, was not merely a theory of descent. This 

 had suggested itself to naturalists in the way I have indicated long before. 

 What Mr. Darwin did was to show how by perfectly natural causes the separation 

 of living organisms into races which at once resemble and yet differ from one 

 another so profoundly came about. Heredity explains the resemblance ; Mr. 

 Darwin's great discovery was that variation worked upon by natural selection 

 explained the difference. That explanation seems to me to gather strength every 

 day and to continually reveal itself as a more and more efficient solvent of the 

 problems which present themselves to the student of natural history. At the 

 same time I am far from claiming for it the authority of a scientific creed or even 

 the degree of certainty which is possessed by some of the laws of astronomy. I 

 only aifirm that as a theory it has proved itself a potent and invaluable instrument 

 of research. It is an immensely valuable induction ; but it has not yet reached 

 such a position of certitude as has been attained by the Law of Gravitation, and I 

 have myself, in the field of botany, felt bound to protest against conclusions being 

 drawn deductively from it without being subjected to the test of experimental 

 verification. This attitude of mine, which I believe I share with most naturalists, 

 must not, however, be mistaken for one of doubt. Of doubt as to the validity of 

 Mr. Darwin's views I have none ; I shall continue to have none till I come across 

 facts which suggest doubt. But that is a different position from one of absolute 

 certitude. 



It is therefore without any dissatisfaction that I observe that many competent 

 persons have, while accepting Mr. Darwin's theory, set themselves to criticise 

 various parts of it. But I must confess that I am disposed to share the opinion 

 expressed by Mr. Huxley, that these criticisms really rest on a want of a thorough 

 comprehension. 



Mr. Romanes has put forward a view which deserves the attention due to the 

 speculations of a man of singular subtlety and dialectic skill. He has startled us 

 with the paradox that Mr. Darwin did not after all put forth, as I conceive it was 

 his own impression he did, a theory of the origin of species, but only of adaptations. 



T Y 2 



