Sept. 13, 1888] 



NATURE 



477 



Though it is difficult to establish the fact that external causes 

 promote variation directly, it is worth considering whether 

 they may not do so indirectly. Weismann. like Lamarck before 

 him, has pointed out, as others have also done, the remarkable 

 persistence of the plants and animals of Egypt ; and the evi- 

 dence of this is now even stronger. We, at Kew, owe to the 

 kindness of Dr. Schweinfurth, a collection of specimens of plants 

 from Egyptian tombs, which are said to be as much as 4000 

 years old. They are still perfectly identifiable, and, as one of 

 coy predecessors in this chair has pointed out, they differ in no 

 respect from their living representatives in Egypt at this day. 

 The explanation which Lamarck gave of this fact " may well," 

 says Sir Charles Lyell, " lay claim to our admiration." He 

 attributed it, in effect, to the persistence of the physical geo- 

 graphy, temperature, and other natural conditions. The ex- 

 planation seems to me adequate. The plants and animals, we 

 may fairly assume, were, 4000 years ago, as accurately adjusted 

 to the conditions in which they then existed as the fact of their 

 persistence in the country shows that they must be now. Any 

 deviation from the type that existed then would either, there- 

 fore, be disadvantageous or indifferent. In the former case it 

 would be speedily eliminated, in the latter it would be swamped 

 by cross-breeding. But we know that if seeds of these plants 

 were introduced into our gardens we should soon detect varieties 

 amongst their progeny. Long observation upon plants under 

 cultivation has always disposed me to think that a change of 

 external conditions actually stimulated variation, and so gave 

 natural selection wider play and a better chance of re-establish- 

 ing the adaptation of the organism to them. Weismann ex- 

 plains the remarkable fact that organisms may for thousands of 

 years reproduce themselves unchanged by the principle of the 

 persistence of the germ-p!asm. Yet it seems hard to believe 

 that the germ-plasm, while enshrined in the individual whose race 

 it is to perpetuate, and nourished at its expense, can be wholly 

 indifferent to all its fortunes. It may be so, but in that case it 

 would be very unlike other living elements of organized beings. 



I am bound, however, to confess that I am not wholly satis- 

 fied with the data for the discussion of this question which 

 practical horticulture supplies. That the contents of our gardens 

 do exhibit the re-ults of variation in a most astonishing degree 

 no one will dispute. But for scientific purposes any exact 

 account of the treatment under which these variations have 

 occurred is unfortunately usually wanting. A great deal of the 

 most striking variation is undoubtedly due to wide crossing, and 

 these cases must, of course, be eliminated when the object is to 

 test the independent variation of the germ-plasm. Hoffmann, 

 whose experiments I have already referred to, doubts whether 

 plants do as a matter of fact vary more under cultivation than in 

 their native home and under natural conditions. It would be 

 very interesting if this could be tested by the concerted efforts 

 of two cultivators, say, for example, in Egypt and in England. 

 Let some annual plant be selected, native of the former country, 

 and let its seed be transmitted to the latter. Then let each 

 cultivator select any variations that, arise in regard to some given 

 character ; set to work, in fact, exactly as any gardener would 

 who wanted to "improve'' the plant, but on a preconcerted 

 plan. A comparison of the success which each obtained would 

 be a measure of the effect of the change of the environment on 

 variability. If it proved that, as Hoffmann supposed, the 

 change of conditions did not affect what we may call the rate of 

 variation, then, as Mr. Darwin remarks in writing to Prof. 

 Semper, "the astonishing variations of almost all cultivated 

 plants must be due to selection and breeding from the varying 

 individuals. This idea," he continues, "crossed my mind 

 many years ago, but I was afraid to publish it,, as I thought that 

 people would say, ' How he does exaggerate the importance of 

 selection.' " From an independent consideration of the subject 

 I also find my mind somewhat shaken about it. Yet I feel 

 disposed to say with Mr. Darwin, "I still must believe that 

 changed conditions give the impulse to variability, but that they 

 act in most cases in a very indirect manner." 



Whatever conclusions we arrive at on these points, everyone 

 will agree that one result of the Darwinian theory has been to 

 give a great impulse to the study of organisms, if I may say so, 

 as "going concerns." Interesting as are the problems which 

 the structure, the functions, the affinity, or the geographical dis- 

 tribution of a plant may afford, the living plant in itself is even 

 more interesting still. 



Every organ will bear interrogation to trace the meaning and 

 origin of its form and the part it plays in the plant's economy. 



That there is here an immense field for investigation there can 

 be no doubt. Mr. Darwin himself set us the example in a series 

 of masterly investigations. But the field is well-nigh inex- 

 haustible. The extraordinary variety of form which plants 

 exhibit has led to the notion that much of it may have arisen 

 from indifferent variation. No doubt, as Mr. Darwin has 

 pointed out, when one of a group of structures held together by 

 some morphological or physiological nexus varies, the rest will 

 vary correlatively. One variation then may, if advantageous, 

 become adaptive, while the rest will be indifferent. But it 

 appears to me that such a principle should be applied with the 

 greatest caution ; and from what I have myself heard fall from 

 Mr. Darwin, I am led to believe that in the later years of his 

 life he was disposed to think that every detail of plant structure 

 had some adaptive significance, if only the clue could be found 

 to it. As regards the forms of flowers an enormous body of in- 

 formation has been collected, but the vegetative organs have not 

 yielded their secret to anything like the same extent. My own 

 impression is that they will be found to be adaptive in innu- 

 merable ways which at present are not even suspected. At Kew 

 we have probably a larger number of species assembled together 

 than are to be found anywhere on the earth's surface. Here T 

 then, is ample material for observation and comparison. But 

 the adaptive significance will doubtless often be found by no- 

 means to lie on the surface. Who, for example, could possibly 

 have guessed by inspection the purpose of the glandular bodies 

 on the leaves of Acacia sphatrocephala and on the pulvinus of 

 Cecropia peltata which Belt in the one case, and Fritz Midler in 

 the other, have shown to serve as food for ants ? So far from 

 this explanation being far-fetched, Belt found that the former 

 "tree is actually unable to exist without its guard," which it 

 could not secure without some attraction in the shape of food. 

 One fact which strongly impresses me with a belief in the adap- 

 tive significance of vegetative characters is the fact that they 

 are constantly adopted in almost identical forms by plants of 

 widely different affinity. If such forms were without significance- 

 one would expect them to be infinitely varied. If, however, 

 they are really adaptive, it is intelligible that different plants 

 should independently avail themselves of identical appliances 

 and expedients. 



Although this country is splendidly equipped with appliances 

 for the study of systematic botany, our Universities and Colleges 

 fall far behind a standard which would be considered even 

 tolerable on the Continent in the means of studying morpho- 

 logical and physiological botany or of making researches in 

 these subjects. There is not at the moment anywhere in 

 London an adequate botanical laboratory, and though at most 

 of the Universities matters are not quite so bad, still I am not 

 aware of any one where it is possible to do more than give the 

 routine instruction, or to allow the students, when they have 

 passed through this, to work for themselves. It is not easy to- 

 see why this should be, because on the animal side the accom- 

 modation and appliances for teaching comparative anatomy and 

 physiology are always adequate and often palatial. Still less- 

 explicable to me is the tendency on the part of those who have 

 charge of medical education to eliminate botanical study from 

 the medical curriculum, since historically the animal histologists 

 owe everything to botanists. In the seventeenth century, as I 

 have already mentioned, Hooke first brought the microscope to 

 the investigation of organic structure, and the tissue he examined 

 was cork. Somewhat later, Grew, in his " Anatomy of Plants," 

 gave the first germ of the cell-theory. During the eighteenth 

 century the anatomists were not merely on a hopelessly wrong, 

 tack themselves, but they were bent on dragging botanists into 

 it also. It was not till 1837, a little more than fifty years ago, 

 that Henle saw that the structure of epithelium was practically 

 the same as that of the parenchyma plantarum which Grew had 

 described 150 years before. Two yea*-s later Schwann pub- 

 lished his immortal theory, which comprised the ultimate facts 

 of plant and animal anatomy under one view. But it was to a 

 botanist, Von Mohl, that, in 1846, the biological world owed 

 the first clear description of protoplasm, and to another botanist,. 

 Cohn (1851), the identification of this with the sarcode of 

 zoologists. 



Now the historic order in discovery is not without its sig- 

 nificance. The path which the first investigators found most 

 accessible is doubtless that which beginners will also find easiest 

 to tread. I do not myself believe that any better access can be 

 obtained to the structure and functions of living tissues than by 

 the study of plants. However, I am not without hopes that the 



