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NATURE 



[January 21, 19 15 



of this step first came home to us in the early 'nineties 

 with Driesch's memorable discovery that by a simple 

 mechanical operation we can at will cause one egg to 

 produce two, or even more than two, perfect embryos. 

 I will not pause to inquire why this result should 

 have seemed so revolutionary. It was as if the scales 

 had fallen from our eyes. With almost a feeling of 

 shock we took the measure of our ignorance and saw 

 the whole problem of development reopened. 



The immediate and most important result of this 

 was to stimulate a great number of important objective 

 investigations in embryology. But let me pause for 

 a moment to point out that at nearly the same time 

 a similar reawakening of interest in the experimental 

 investigation of problems of the present became 

 evident in many other directions — for example, in 

 studies on growth and regeneration ; on cytology and 

 protozoology ; on economic biology ; on ecology, the 

 behaviour of animals and their reactions to stimuli ; 

 on heredity, variation, and selection. The leaven was 

 indeed at work in almost every field of zoology, and 

 everywhere led to like results. It was a day of rapid 

 obliteration of conventional boundary lines ; of revolt 

 from speculative systems towards the concrete and 

 empirical methods of the laboratory ; of general and 

 far-reaching extension of experimental methods in our 

 science. 



But I will return to embryology. It may be doubted 

 whether any period in the long history of this science 

 has been more productive of varied and important 

 discoveries than that which followed upon its adoption 

 of experimental methods. In one direction the em- 

 bryologist went forward to investigations that brought 

 him into intimate relations with the physicist, the 

 chemist, the pathologist, and even the surgeon. A 

 flood of light was thrown on the phenomena of 

 development by studies on differentiation, regeneration, 

 transplantation, and grafting ; on the development of 

 isolated blastomeres and of egg-fragments ; on the 

 symmetry and polarity of the &^g\ on the relations of 

 development to mechanical, physical, and chemical 

 conditions in the environment ; on isolated living cells 

 and tissues, cultivated like micro-organisms, outside 

 the body in vitro ; on fertilisation, artificial partheno- 

 genesis, and the chemical physiology of development. 

 In respect to the extension of our real knowledge these 

 advances constitute an epoch-making gain to biological 

 science. And yet these same researches afford a most 

 interesting demonstration of how the remoter problems 

 of science, like distant mountain-peaks, seem to recede 

 before us even while our actual knowledge is rapidly 

 advancing. Thirty years after Roux's pioneer re- 

 searches we find ourselves constrained to admit that in 

 spite of all that we have learned of development the 

 egg has not yet yielded up its inmost secrets. I have 

 referred to the admirable discovery of Driesch con- 

 cerning the artificial production of twins. That bril- 

 liant leader of embryological research had in earlier 

 years sought for an understanding of development 

 along the lines of the mechanistic or physico-chemical 

 analysis, assuming the G.g^ to be essentially a physico- 

 chemical machine. He now admitted his failure and, 

 becoming at last convinced that the quest had from 

 the first been hopeless, threw all his energies into an 

 attempt to resuscitate the half-extinct doctrines of 

 vitalism and to found a new philosophy of the organ- 

 ism. Thus the embryologist, starting from a simple 

 laboratory experiment, strayed further and further 

 from his native land until he found himself at last 

 quite outside the pale of science. He did not always 

 return. Instead, he sometimes made himself a new 

 home — upon occasion even established himself in the 

 honoured occupancy of a university chair of 

 philosophy ! 



NO. 2360, VOL. 94] 



The theme that is here suggested tempts me to a 

 digression, because of the clear light in which it dis- 

 plays the attitude of modern biology towards the study 

 of living things. It is impossible not to admire the 

 keenness of analysis, and often the artistic refinement 

 of skill (which so captivates us, for instance, in the 

 ' work of M. Bergson) with which the neo-vitalistic 

 j writers have set forth their views. For my part, I 

 ' am ready to go further, admitting freely that the 

 I position of these writers tnay at bottom be well 

 \ grounded. At any rate, it is well for us now and 

 ' then to be rudely shaken out of the ruts of our accus- 

 tomed modes of thought by a challenge that forces 

 upon us the question whether we really expect our 

 scalpels and microscopes, our salt-solutions, formulas, 

 and tables of statistics, to tell the whole story of 

 living things. It is, of course, impossible for us to 

 assert that they will. And j^et the more we ponder the 

 question the stronger grows our conviction that the 

 " entelechies " and such-like agencies conjured forth 

 by modern vitalism are as sterile for science as the 

 final causes of an earlier philosophj- ; so that Bacon 

 I might have said of the former, as he did of the latter, 

 i that they are like the Vestal virgins — dedicated to 

 God, and barren. We must not deal too severely with 

 the naturalist who now and then permits himself an 

 hour of dalliance with them. An uneasy conscience 

 will sooner or later drive him back into his own 

 straight and narrow way with the insistent query : 

 The specific vital agents, sui generis, that are postu- 

 lated by the vitalist — are they sober realities? Can the 

 existence of an "elan vital," of "entelechies," of 

 "psychoids," be experimentally verified? Even if 

 beyond the reach of verification may they still be of 

 practical use in our investigations on living things, 

 or find their justification on larger grounds of scien- 

 tific expediency. However philosophy may answer, 

 science can find but one reply. The scientific method 

 is the mechanistic method. The moment we swerve 

 from it by a single step we set foot in a foreign land 

 where a different idiom from ours is spoken. We 

 have, it is true, no proof whatever of its final validity. 

 We do not adopt the mechanistic view of organic 

 nature as a dogma but only as a practical programme 

 of work, neither more nor less. We know full well 

 that our present mechanistic conceptions of animals 

 and plants have not vet made any approach to a com- 

 plete solution of the problems of life, whether past or 

 present. This should encourage us to fresh efforts, 

 for just in the present inadequacy of these conceptions 

 lies the assurance of our future progress. But the 

 way of unverifiable (and irrefutable) imaginative con- 

 structions is not our way. We must hold fas.t to the 

 method by which all the great advances in our know- 

 ledge of nature have been achieved. We shall make 

 lasting progress onlv by plodding along the old, hard 

 beaten trail blazed by our scientific fathers — the way 

 of observation, comparison, experiment, analysis, syn- 

 thesis, prediction, verification. If this seems a prosaic 

 programme we may learn otherwise from great dis- 

 coverers in every field of science who have demon- 

 strated how free is the play that it gives to the con- 

 structive imagination and even to the faculty of artistic 

 creation. 



II. 

 Thus far I have desired to emphasise especially the 

 reawakening of our interest in problems of the present, 

 and the growing importance of experimental methods 

 in our science. It is interesting to observe how these 

 changes have affected our attitude towards the his- 

 torical problem as displayed in the modern study of 

 genetics. Even here we are struck by the same shift- 

 ing of the centre of gravity that has been remarked 

 in other fields of inquiry. In the Darwinian era 



