BRITISH ASSOCIATION : ZOOLOGICAL ADDRESS. 349 



precise knowledge of the steps by which these adult relations are 

 established during the development of different kinds of animals ; on 

 our constantly increasing knowledge of the succession of animal 

 forms in past time ; and, generally, on the conviction that all the 

 diverse forms of tissues, organs, and entire animals are but the 

 expression of an infinite number of variations of a single theme, that 

 theme being cell-division, multiplication, and differentiation. This 

 conviction grew but slowly in men's minds. It was opposed to the 

 cherished beliefs of centuries, and morphology rendered a necessary 

 service when it spent all those years which have been described as 

 " years in the wilderness " in accumulating such a mass of circum- 

 stantial evidence in favour of an evolutionary explanation of the 

 order of animate nature as to place the doctrine of descent with 

 modification on a secure foundation of fact. I do not believe that 

 this foundation could have been so securely laid in any other way, 

 and I hold that zoologists were actuated by a sound instinct in 

 working so largely on morphological lines for forty years after 

 Darwin wrote. For there was a large mass of fact and theory to be 

 remodelled and brought into harmony with the new ideas, and a still 

 larger vein of undiscovered fact to explore. The matter was difficult 

 and the pace could not be forced. Morphology, therefore, deserves 

 the credit of having done well in the past : the question remains, 

 What can it do in the future ? 



It is evident, I think, that it cannot do much in the way of 

 adding new truths and general principles to zoological science, nor 

 even much more that is useful in the verification of established 

 principles, without enlarging its scope and methods. Hitherto — or, 

 at any rate, until very recently — it has accepted certain guiding 

 principles on faith, and, without inquiring too closely into their 

 validity, has occupied itself with showing that, on the assumption 

 that these principles are true, the phenomena of animal structure, 

 development, and succession receive a reasonable explanation. 



We have seen that the fundamental principles relied upon during 

 the last fifty years have been inheritance and variation. In every 

 inference drawn from the comparison of one kind of animal structure 

 with another, the morphologist founds himself on the assumption 

 that different degrees of similitude correspond more or less closely 

 to degrees of blood-relationship, and to-day there are probably few 

 persons who doubt that this assumption is valid. But we must not 

 forget that, before the publication of the ' Origin of Species,' it was 

 rejected by the most influential zoologists as an idle speculation, 

 and that it is imperilled by Mendelian experiments showing that 

 characters may be split up and reunited in different combinations in 

 the course of a few generations. We do not doubt the importance 

 of the principle of inheritance, but we are not quite so sure as we 

 were that close resemblances are due to close kinship and remoter 

 resemblances to remoter kinship. 



The principle of variation asserts that like does not beget exactly 

 like, but something more or less different. For a long time morpho- 

 logists did not inquire too closely into the question how these 

 differences arose. They simply accepted it as a fact that they occur, 



