NA TURE 



237 



THURSDAY. DECEMBER 21, 191 1. 



NOTES OF A NATURALIST. 



Convergence in Evolution. By Dr. A. Willey, F.R.S. 

 Pp. xiv+177. (London: John Murray, 1911.) 

 Price js. 6d. net. 



DR. WILLEY has had great opportunities of 

 studying animal life under varied conditions, 

 and he has given us in this interesting little volume 

 some of the fruits of his observations. Here, in brief 

 compass, will be found a zoologist's notes — field-notes 

 nnd laboratory-notes — on points which have specially 

 attracted him. Indeed, we think that some title sug- 

 gesting that the book contained the varied observations 

 of a zoologist would have been preferable to that which 

 has been chosen. The book has been carefully edited 

 and is well and clearly printed. One very conspicuous 

 slip has, however, escaped notice, the reference to 

 p. 90 instead of to 94 on the frontispiece itself. 



The author tells us in the preface that he uses " the 

 word convergence in a wide sense, embracing habits, 

 functions, structures, and physiognomy." The sense 

 is indeed so wide that the force of the term becomes 

 attenuated. Thus even the convenient Box-and-Cox 

 sleeping arrangements of the fruit-eating bats and 

 crows in the maritime districts of Ceylon are classed 

 as an instance " of convergent homing, the same trees 

 affording hospitality in regular succession to day- 

 Hying birds and night-flying mammals " (p. 25). 



Speaking of the well-known Kallima butterflies, the 

 f rdinary representation of the attitude is corrected, 

 and it is shown that, at least in the Ceylon species 

 (/v. philarchus), the insect rests head downwards — a 

 (act which has also been recorded of the Indian K. 

 inachis. Concerning "the extreme amount of indi- 

 vidual variation in the markings on the under side of 

 wings, simulating all degrees of decay and discolora- 

 tion and fungus attack" (p. 58), the author speaks 

 with somewhat unnecessary respect of the notion 



that the constant repetition of such considerable 

 variations as are met with in leaf-like Lepidoptera 

 and Orthoptera, from generation to generation, is a 

 standing witness against the truth of ' Darwinism,' 

 inasmuch as, according to the Darwinian theory, such 

 variations ought either to become fixed by natural 

 selection or swamped by interbreeding" (p. 61). 



The tentative suggestion on p. 61, "it may be that 

 natural selection is interested in keeping alive the 

 variations for the benefit of the species, not for the 

 production of new species," may be accepted with some 

 confidence when such polymorphic forms are looked 

 at as a whole. If it be an advantage in the struggle 

 for existence to resemble a dead leaf or another butter- 

 fly of a distasteful group, it is clearly a still greater 

 advantage to resemble two or more kinds of dead leaf, 

 or two or more unpalatable "models." It must be 

 remembered that Kallima is by no means remarkable 

 in this respect, for such polymorphism is well known 

 in immense numbers of both procryptic and mimetic 

 species. 



It is satisfactory to find that such an extreme sup- 

 NO. 2199, VOL. 88] 



porter of the far-reaching influence of convergence 

 does not, at any rate for the present, assail the Dar- 

 winian conclusion that the same specific form is never 

 repeated a second time as the outcome of an inde- 

 pendent line of evolution. 



"The present state of our knowledge," he 

 writes on p. 138, "justifies the provisional 

 assertion that the higher combination which 

 leads to the establishment of an animal form possess- 

 ing the essential component elements of a definite 

 morphological type, cannot be repeated. The theory 

 of convergence is therefore not calculated to precipi- 

 tate us into morphological chaos, howsoever startling 

 its manifestations may be." 



The author is certainly prepared to be startling, as, 

 for instance, in his conclusion that the closed 

 nephridia as a whole, flame-cells (or solenocytes) and 

 all, arose independently in the Pol5Thaete worms and 

 in Amphioxus. But before adopting any such hypo- 

 thesis, it is prudent to investigate the evidence that 

 these structures were present in the probable common 

 ancestor of both Amphioxus and Polychasta. That 

 such nephridia did thus exist in the primitive stock 

 is strongly supported by their occurrence in several 

 other groups which are much nearer to the common 

 ancestor than either Amphioxus or the Polychaeta. 

 We have reason to believe that E. S. Goodrich, who 

 originally discovered the resemblance in minute detail 

 between the solenocytes of these two forms, considers 

 that his conclusions have been much strengthened by 

 the results of investigations on other groups of the 

 Coelomata. 



There can be no doubt, however, that the inde- 

 pendent origin of elaborate structures has occurred 

 again and again. There is scarcely a subject in which 

 it is more necessary to bear in mind the commonplace 

 saying that every case must be argued on its merits 

 without any bias in favour of one interpretation rather 

 than another. 



An excellent example of convergence in a structure 

 of considerable complexity was brought forward more 

 than thirty years ago by Fritz Miiller. At the same 

 time, we must remember that the male scent-organs 

 on the wings of butterflies, to which he was referring, 

 are constructed of modified scales — elements which 

 are so excessively variable in size, form, and structure 

 that the independent appearance of anything that can 

 be produced by a scale is probably easier than almost 

 any other feat of convergent evolution. Speaking of 

 the scent-organs on the wings of certain male Satyrine 

 and Nymphaline butterflies, Fritz Miiller wrote : — 



" I know of no other case which proves so clearly 

 and irrefragably, and attests with such force, the truth 

 of a principle which should never be lost sight of in 

 morphological studies. When in two species certain 

 organs which serve the same function are found in 

 the same place, are composed of the same parts, 

 occupying the same relative positions, and exhibiting 

 similar forms — all this by itself constitutes no sufficient 

 proof that these organs are homologous, nor does it 

 afford the grounds for including the two sprries in 

 the same familv." ' 



E. B. P. 



• Mrc/t. Mus. N.1C. Rio de Janeiro, iii. (1878), pp. 1-7. 



