38o 



NATURE 



[September 22, 1910 



school of experimental embryology has outgrown its infancy 

 and has developed into a vigorous youth. It has produced 

 some very remarkable results, which cannot fail to exercise 

 a lasting influence on the course of zoological studies. We 

 have learnt from it a number of positive facts, from which 

 we may draw very important conclusions, subversive of 

 some of the most cherished ideas of whilom morphologists. 

 Jt has been proved by experiment that very small changes 

 in the chemical and physical environment may and do 

 produce specific form-changes in developing organisms, and 

 in such experiments the consequence follows so regularly 

 on the antecedent that we cannot doubt that we have true 

 relations of cause and effect. It is not the least interesting 

 outcome of these experiments that, as Loeb has remarked, 

 it is as yet impossible to connect in a rational way the 

 effects produced with the causes which produced them, and 

 it is also impossible to define in a simple way the character 

 of the change so produced. For example, there is no 

 obvious connection between the minute quantity of sulphates 

 present in sea-water and the number and position of the 

 characteristic calcareous spicul&s in the larva of a sea- 

 urchin. Yet Herbst has shown that if the eggs of sea- 

 urchins are reared in sea-water deprived of the needful 

 sulphates (normally o'26 per cent, magnesium sulphate 

 and oi per cent, calcium sulphate), the number and 

 relative positions of these spicules are altered, and, in 

 addition, changes are produced in other organs, such as the 

 gut and the ciliated bands. Again, there is no obvious 

 connection between the presence of a small excess of 

 magnesium chloride in sea-water and the development of 

 the paired optic vesicles. Yet Stockard, by adding 

 magnesium chloride to sea-water in the proportion of 

 6 grams of the former to loo c.c. of the latter, has produced 

 specific effects on the eyes of developing embryos of the 

 minnow Fundulus hctt'roclitus : the optic vesicles, instead 

 of being formed as a widely separated pair, were caused to 

 approach the median line, and in about 50 per cent, of 

 the embryos experimented upon the changes were so pro- 

 found as to give rise to cyclopean monsters. Many other 

 instances might be cited of definite effects of physical and 

 chemical agencies on particular organs, and we are now 

 forced to admit that inherited tendencies may be completely 

 overcome by a minimal change in the environment. The 

 nature of the organism, therefore, is not all important, 

 since it yields readily to influences which at one time we 

 should have thought inadequate to produce perceptible 

 changes in it. 



It is open to anyone to argue that, interesting as experi- 

 ments of this kind may be, they throw no light on the 

 origin of permanent — that is to say, inheritable — modifica- 

 tions of structure. It has for a long time been a matter 

 of common knowledge that individual plants and animals 

 react to their environment, but the modifications induced 

 Tjy these reactions are somatic ; the germ-plasm is not 

 affected, therefore the changes are not inherited, and no 

 permanent effect is produced in the characters of the race 

 or species. It is true that no evidence has yet been pro- 

 ■duced to show that form-changes as profound as those that 

 I have mentioned are transmitted to the offspring. So far 

 the experimenters have not been able to rear the modified 

 organisms beyond the larval stages, and so there are no 

 offspring to show whether cyclopean eyes or modified forms 

 of spicules are Inherited or not. Indeed, it is possible that 

 the balance of organisation of animals thus modified has 

 been upset to such an extent that they are incapable of 

 ■growing into adults and reproducing their kind. 



But evidence is beginning to accumulate which shows 

 that external conditions may produce changes in the germ- 

 ■cells as well as in the soma, and that such changes may be 

 ■specific and of the same kind as similarly produced somatic 

 changes. Further, there is evidence that such germinal 

 ■changes are inherited — and, indeed, we should expect them 

 to be, because they are germinal. 



The evidence on this subject is as yet meagre, but it is 

 of good quality and comes from more than one source. 



There are the well-known experiments of Weismann, 

 Standfuss, Mcrrifield, and E. Fischer on the modification 

 of the colour patterns on the wings of various Lepidoptera. 

 In the more northern forms of the fire-butterfly, Chryso- 

 phanus (Polyommatus) pltlaeas, the upper surfaces of the 

 wings arc of a bright red-gold or copper colour with a 

 narrow black margin, but In southern Europe the black 

 NO. 2134, VOL. 84I 



tends to extend over the whole surface of the wing and 

 may nearly obliterate the red-gold colour. By exposing 

 pup.-E of caterpillars collected at Naples to a temperature 

 of 10° C. Weismann obtained butterflies more golden than 

 the Neapolitan, but blacker than the ordinary German 

 race, and conversely, by exposing pupa^ of the German 

 variety to a temperature of about 38° C, butterflies were 

 obtained blacker than the German, but not so black as the 

 Neapolitan variety. Similar deviations from the normal 

 standard have been obtained by like means in various 

 species of Vanessa by Standfuss and Merrifield. Standfuss, 

 working with the small tortoiseshell butterfly {Vanessa 

 urticac), produced colour aberrations by subjecting the 

 pupa; to cold, and found that some specimens reared under 

 normal conditions from the eggs produced by the aberrant 

 forms exhibited the same aberrations, but in a lesser 

 degree. Weismann obtained similar results with the same 

 species. E. Fischer obtained parallel results with .4rf(ia 

 caja, a brightly coloured diurnal moth of the family 

 Bombycid^. Pupa; of this moth were exposed to a tem- 

 perature of 8° C, and some of the. butterflies that emerged 

 were very dark-coloured aberrant forms. A pair of these 

 dark aberrants were mated, and the female produced eggs, 

 and from these larvae and pupae were reared at a normal 

 temperature. The progeny was for the most part normal, 

 but some few Individuals exhibited the dark colour of the 

 parents, though in a less degree. The simple conclusions 

 to be drawn from the results of these experiments is that 

 a proportion of the germ-cells of the animals experimented 

 upon were affected by the abnormal temperatures, and that 

 the reaction of the germ-cells was of the same kind as the 

 reaction of the somatic cells and produced similar results. 

 As everybody knows, Weismann, while admitting that the 

 germ-cells were affected, would not .-idmit the simple 

 explanation, but gave another complicated and, in my 

 opinion, wholly unsupported explanation of the phenomena. 

 In any case this series of experiments was on too small 

 a scale, and the separate experiments were not sufficiently 

 carefully planned to exclude the possibility of error. But 

 no objection of this kind can be urged against the careful 

 and prolonged studies of Tower on the evolution ot 

 chrysomclid beetles of the genus Lcptinotarsa. Leptino- 

 tarsa — better known, perhaps, by the name Doryphora — ii 

 the potato-beetle, which has spread from a centre in North 

 Mexico southwards into the Isthmus of Panama and north- 

 wards over a great part of the United States. It is 

 divisible into a large number of species, some of which ar? 

 dominant and widely ranging ; others are restricted to very 

 small localities. The specific characters relied upon are 

 chiefly referable to the coloration and colour patterns of 

 the epicranium, pronotum, elytra, and underside of the 

 abdominal segments. In some species the specific markings 

 are very constant, in others, particularly in the common 

 and wide-ranging L. decenilineata, they vary to an extreme 

 degree. As the potato-beetle is easily reared and main- 

 tained in captivity, and produces two broods every year, it 

 is a particularly favourable subject for experimental investi- 

 gation. Tower's experiments have extended over a period 

 of eleven years, and he has made a thorough study of the 

 geographical distribution, dispersal, habits, and natural 

 history of the genus. The w-hole work appears to have 

 been carried out with the most scrupulous regard to 

 scientific accuracy, and the author is unusually cautious in 

 drawing conclusions and chary of offering hypothetical 

 explanations of his results. I have been greatly impressed 

 by the large scale on which the experiments have been 

 conducted, by the methods used, by the care taken to 

 verify every result obtained, and by the great theoretical 

 importance of Tower's conclusions. I can do no more now 

 than allude to some of the most remarkable of them. 



After showing that there are good grounds for believing 

 that colour production in insects is dependent on the action 

 of a group of closely related enzymes, of which chitase, the 

 agent which produces hardening of chitin, is the most 

 important. Tower demonstrates by a series of well-planned 

 expcrinients that colours are directly modified by the action 

 of external agencies — viz., temperature, humidity, food, 

 altitude, and light. Food chiefly affects the subhypodermal 

 colours of the larvie, and does not enter much into account; 

 the most important agents affecting the .adult color.atlon 

 being temperature and humidity. .'\ sligJit increase or a 

 slit;ht decrease of temperature or humidity was found to 



