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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 

 wore 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. Difficulties and objections were 

 raised, but morphology on the whole took little heed of them. Remaining stead- 

 fast in its adherence to the principles 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 in- 

 stances, and especially such instances as seem 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 indis- 

 pensable 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. 



As a matter of fact, admitting that the old rules are valid, it has become in- 

 creasingly evident that they are not sufficient. Until a few years ago morpho- 

 logists were open to the reproach that, while they studied form in all its variety 

 and detail, they occupied themselves too little — if, indeed, they 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 Hof- 

 meister, as long ago as 1868, wrote that the most pressing and immediate aim of 

 the investigator was to discover 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 descriptive science, if it was 

 to progress any further in the discovery of the relations of cause and effect, it 

 was clear that it must alter its methods and follow the course indicated by 

 Hofmeister. And I submit that an inquiry into the causes which produce altera- 

 tion 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 molar teeth of 

 mammals or a disputation about the origin of the ccelomic cavities of vertebrated 

 and invertebrated 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 physiological units, of micellae, 

 of determinants and biophors, or of pangenes ; all of them essentially morpholo- 

 gical conceptions; the products of imagination projected beyond the confines of the 

 visible, yet always restrained by having only one source of experience — 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 representations of 

 the genesis of organic structure, and yet recognise that their chief utility has been 

 to make us realise more clearly 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 



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