420 



NATURE 



[June 2, 192 1 



sward from the front of the snout upwards, while 

 the cat dresses her snout downwards with her 

 paw. We should think this was a hysteron pro- 

 teron — the cart before the horse. The dog has 

 on his chest a reversed area of hair — spreading 

 out on each side. "When lying with his head 

 supported on his paws the lower part of his chest 

 is closely applied to the upper or flexor surface of 

 the fore legs, and the long-continued pressure of 

 the latter against the downward or normal streams 

 of hair on the chest leads to its slope being 

 reversed." The downward slope of the shaggy 

 hairs of the two-toed sloth, that spends so much 

 of its time upside down below the branches, is to 

 be attributed to the action of gravity upon the 

 long hairs. We wonder that the author does not 

 allude to the downward pull which the coating of 

 green Algae on the hairs must involve ! 



The factors recognised by Dr. Kidd in the 

 formation of hair-patterns are four : friction, pres- 

 sure, gravity, and underlying muscular traction. 

 His thesis is that changes in the conditions of life 

 — e.g. in modes of locomotion and in attitudes of 

 rest — have directly brought about modifications 

 in the lie of the hair, and that these modifications 

 have been cumulatively entailed on the race. 



" Initiative in animal evolution comes by stimu- 

 lation, excitation, and response in new conditions, 

 and is followed by repetition of these phenomena 

 until they result in structural modifications, trans- 



itted and directed by selection and the laws of 

 genetics — a series of events which agree with Neo- 

 Lamarckian principles." 



Now it is familiarly easy for Lamarckians to 

 interpret structural peculiarities as the outcome of 

 transmitted exogenous modifications (the direct 

 somatic results of peculiarities in function, habit, 

 nutrition, and environment), and Dr. Kidd is 

 much too thoughtful an investigator to be con- 

 tent with mere interpretation. He brings forward 

 evidence to show that the lie of the hair can be 

 modified in the individual; he also brings forward 

 some evidence to show that parental modifications 

 may reappear in the offspring. Speaking frankly, 

 we do not think the evidence is strong, but it is 

 progress to have any evidence at all. 



As regards individual modifications of the hair- 

 pattern, reference is made to the way in which the 

 peculiar functioning of the muscles in the vicinity 

 of the human eyebrow alters its shape and char- 

 acter. 



" It is shown by numerous examples in the 

 human eyebrow that the muscles underneath the 

 hairs which are embedded in the true skin for a 

 tangible depth, do play havoc with the normal 

 arrangement of hair, as the conflict proceeds, the 

 resultant ' pull ' being actually engraved, signed 



NO. 2692, VOL. 107] 



and sealed by physiological wrinkles of the fore- 

 head and face." 



It may be so, but one must tread warily. There 

 are individual idiosyncrasies in the eyebrows 

 hinted at very early in life, long before the time 

 of wrinkling, which become emphasised when the 

 eyebrow hairs grow longer, as they so often do 

 in later life. First catch your modification. 



A little more cogent, it seems to us, is the 

 chapter on the modifications in the hair-patterns 

 of horses, modifications which can be traced to 

 peculiarities of harness. Yet here again there is 

 need for scepticism. One has to be sure that the 

 peculiarities observed are not constitutional varia- 

 tions, quite independent of harness ; and here one 

 must go back in Dr. Kidd's book to the excellent 

 treatment of the whorls, featherings, and crests 

 that frequently occur on the horse's neck, most of 

 which cannot, without great difficulty, be regarded 

 as modifications. Moreover, one has to remember 

 that in a hard-worked horse there may be a 

 coercive reversal of the moist hair, which never 

 gets a chance to right itself, and is not, therefore,. 

 a true modification which persists after the in- 

 ducing factors have ceased to operate. As to the 

 ten foals showing a reversed area or pattern on 

 the under surface of their necks similar to that 

 which their mothers showed, we wish to be sure 

 that the maternal reversal was due to the collar. 

 But of the value of collecting cases like this^ 

 even if they do not convince sceptics, there can 

 be no doubt, and Dr. Kidd will be thanked by all 

 biologists. 



Dr. Kidd supports his case with facts relating 

 to the formation of new bursas under the stress of 

 mechanical forces and to Pavlov's experimental 

 production of new reflexes in the life of the in- 

 dividual, but he stakes his argument on the lie of 

 the hair. His general position is that initiatives 

 or new departures in evolution are direct answers 

 to peculiarities in nurture (activity, rest, food, and 

 environment), and that these answers are trans- 

 missible in a representative way which becomes 

 cumulative, unless, indeed, selection intervenes. 

 He coins the word "plasto-diethesis," combining 

 the metaphors of mould and sieve; the organism 

 is moulded in some new way by peculiarities in 

 function and environment, ^ttd the moulded 

 organisms are sifted. "So the banns between 

 Lamarck and Darwin are published, not for the 

 first time of asking, and who shall say that there 

 is cause or just impediment why these two should 

 not be joined in holy matrimony? " 



We suspect that the lie of the hair is fixedly 

 determined by the slope of the hair follicle beneath 

 the surface of the skin, and that this slope, though 



