September 22, 1910] 



NATURE 



379 



lof<ical facts were consistent with it. We are apt to forget, 

 nowadays, that there is no a priori reason for regarding 

 the resemblances and differences that we observe in organic 

 forms as something different in kind from the analogous 

 series of resemblances and differences that obtain in 

 inanimate objects. This was clearly pointed out by 

 l-'li-c-ming Jenkin in a very able and much-referred to article 

 in the Aor/d liritish Review for June, 1867, and his argu- 

 ment from the a priori standpoint has as much force to-day 

 as when it was written forty-three years ago. But it has 

 lost almost all its force through the arguments a posteriori 

 supplied by morphological science. Our belief in the trans- 

 mutation of animal organisation in past time is founded 

 very largely upon our minute and intimate knowledge of 

 the manifold relations of structural form that obtain among 

 adult animals ; on our precise knowledge of the steps by 

 which these adult relations are established during the 

 development of different kinds of animals; on our con- 

 stantly 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, multipli- 

 cation, 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 circumstantial 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. 1 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, 1 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 verifica- 

 tion of established principles, without enlarging its scope 

 and methods. Hitherto — or, at any rate, until very re- 

 cently — 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 

 \'ariation. In every inference drawn from the comparison 

 of one kind of animal structure with another, the 

 morphologist founds himself on the assumption that dif- 

 ferent degrees of similitude correspond more or less closely 

 to degrees of blood-relationship, and to-day there are prob- 

 ably 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 in- 

 fluential zoologists as an idle speculation, and that it is 

 imperilled by .Mendclian experiments showing that char- 

 acters may be split up .ind reunited in different combina- 

 tions 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 morphologists did not inquire too closely into the 

 question how these differences arose. They simply accepted 

 it as a fact that they occur, and that they are of sufficient 

 frequency and magnitude, and that a sufficient proportion 

 of them lead in such directions that natural selection can 

 take advantage of them. DiflRculties and objections were 

 raised, but morphology on the whole took little heed of 

 them. Remaining steadfast in its adherence to the prin- 



NO. 2134, VOL. 84] 



ciples laid down by Darwin, it contented itself with piling 

 up circumstantial evidence, and met objection and criticism 

 with an ingenious apologetic. In brief, its labours have 

 consisted in bringing fresh instances, and especially such 

 instances as seemed unconformable, under the rules, and 

 in perfecting a system of classification in illustration of 

 the rules. It is obvious, however, that, although this kind 

 of study is both useful and indispensable at a certain stage 

 of scientific progress, it does not help us to form new 

 rules, and fails altogether if the old rules are seriously 

 called into question. 



.'Xs a matter of fact, admitting that the old rules are 

 valid, it has become increasingly evident that they are not 

 sufficient. Until a few years ago morphologists were open 

 to the reproach that, while they studied form in all its 

 variety and detail, they occupied themselves too little — if, 

 indeed, thev could be said to occupy themselves at all — ■ 

 with the question of how form is produced, and how, when 

 certain forms are established, they are caused to undergo 

 change and give rise to fresh forms. As Klebs has pointed 

 out, the forms of animals and plants were regarded as the 

 expression of their inscrutable inner nature, and the stages 

 passed through in the development of the individual were 

 represented as the outcome of purely internal and hidden 

 laws. This defect seems to have been more distinctly 

 realised by botanical than by zoological morphologists, for 

 Hofmeister, as long ago a's 1868, wrote that the most 

 pressing and immediate aim of the investigator was to dis- 

 cover to what extent external forces acting on the organism 

 are of importance in determining its form. 



If morphology was to be anything more than a descrip- 

 tive science, if it was to progress any further in the 

 di.scovery of the relations of cause and effect, it was clear 

 that it ' must alter its methods and follow the course 

 indicated by Hofmeister. .'\nd I submit that an inquiry 

 into the causes which produce alteration of form is as much 

 the province of, and is as fitly called, morphology as, let 

 us say, a discussion of the significance of the patterns of 

 the rnolar teeth of mammals or a disputation about the 

 origin of the coelomic cavities of vertebrated and inverte- 

 brated animals. 



There remains, therefore, a large field for morphology to 

 explore. Exploration has begun from several sides, and in 

 some quarters has made substantial progress. It will be 

 of interest to consider how much progress has been made 

 along certain lines of research — we cannot now follow all 

 the lines — and to forecast, if possible, the direction that 

 this pioneer work will give to the morphology of the future. 



I am not aware that morphologists have, until quite 

 recently, had any very clear concept of what may be 

 expected to underlie form and structure. Dealing, as they 

 have dealt, almost exclusively with things that can be seen 

 or rendered visible by the microscope, they have acquired 

 the habit of thinking of the organism as made up of 

 organs, the organs of tissues, the tissues of cells, and the 

 cells as made up— of what? Of vital units of a lower 

 order, as several very distinguished biologists would have 

 us believe ; of phvsiological units, of micellae, of deter- 

 minants and biophors, or of pangenes ; all of them 

 essentially morphological conceptions ; the products of 

 imagination projected beyond the confines of the visible, 

 yet alwavs restrained by having only one source of experi- 

 ence — namely, the visible. One may give unstinted 

 admiration to the brilliancy, and even set a high value on 

 the usefulness, of these attempts to give formal representa- 

 tions of the genesis of organic structure, and yet recognise 

 that their chief utility has been to make us realise more 

 clearlv the problems that have yet to be solved. 



Stripped of all the verbiage that has accumulated about 

 them, the simple questions that lie immediately before us 

 are : What are the causes which produce changes in the 

 forms of animals and plants? .'\re they purely internal, 

 and, if so, are their laws discoverable? Or are they partly 

 or whollv external, and, if so, how far can we find 

 relations -'of cause and effect between ascertained chemical 

 and physical phenomena and the structural responses of 

 living beings? 



As" an attempt to answer the last of these questions, we 

 have the recent researches of the experimental morpholo- 

 gists and embryologists directed towards the very aim that 

 Hofmeister proposed. Originally founded by Roux, the 



