5^4 



NATURE 



[October i i, 1906 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Biometry and Biology : A Reply to Prof. Pearson. 



In reply to Prof. Pearson's leiler in Xatiri; of 

 September 6 (p. 405;, 1 desire, in the first place, to express 

 my e.xtreme regret if the criticism which 1 ventured to 

 offer on biometrical work in my address at York has 

 caused pain in a quarter where I should least desire to 

 give offence. Had I foreseen that this was likely to 

 happen I certainly should have refrained from making any 

 criticism on that occasion. 



Prof. Pearson wishes me to e.vplain how Dr. Pearl's 

 paper, " A Biometrical .Study of Conjugation in Para- 

 majcii'.rn," an abstract of which appeared in the Proceed- 

 ings of the Royal Society (.B, 518, p. 377), lays him open 

 to the advice that he should make sure that the problem 

 he seeks to elucidate is sound from the standpoint of 

 biology. I think that there is no course open to me but 

 lo comply. 



Dr. Pearl states that his work on Paramecium caudatum 

 was undertaken for the purpose of obtaining answers to 

 the questions : — 



" (a) Is the portion of the Paramjecium population which 

 is in a state of conjugation at a given time differentiated 

 in respect of type or variability, or both, from the non- 

 conjugating portion of the population living in the same 

 culture at the same time? 



" (fc) Is there any tendency for like to pair with like 

 I" homogamy ") in the conjugation of Parania;cium, and if 

 so, how strong is this tendency? " 



In making the first inquiry, and in dealing with it, 

 Dr. Pearl appears to ignore the fact that the differentiation 

 of the conjugants of this species is already well estab- 

 lished. Maupas {.Arch, de Zool. exp., ser. 2, T. vii., p. 

 184), writing in 1889, says : — " Tous les observateurs qui 

 se sont occup^s de la conjugaison du Parameciutn caudatunt 

 ont signal^ la petitesse de taille des individus accoupl^s." 

 He goes on to say that he has never found them to exceed 

 225 fi in length, usuallv 180 /i to 210 /i, while it is not 

 rare to find non-conjugants attaining 300 /i or even 320 fi. ; 

 so that when Dr. Pearson states that " Dr. Pearl demon- 

 strates for the first time that conjugant Paramsecia are 

 differentiated from the non-conjugant population," he 

 appears to be in error. 



It may, however, be claimed that by the application of 

 the biometrical method of dealing with the series of 

 measurements he has given a more precise measure of 

 their differentiation. 



I would .submit that Dr. Pearl's material and modes of 



procedure are singularly unfitted for yielding such a result. 



In the first place, the specimens have been preserved and 



fixed, a process which every practical biologist knows to 



be attended with distortion. 



They were prepared by different hands, partly by Dr. 

 Pearl himself in Leipzig, partly by Prof. Worcester in 

 .\merica. 



Dr. Pearl tells us (p. 377) that " in the measuring 

 conjugant pairs were taken quite at random, and then in 

 each case the two undistorled non-conjugant individuals 

 which were lying nearest in the field of view of the micro- 

 scope to the conjugant pair were measured." 



Now let us consider what would happen with this mode 

 of procedure. Paramecium, as is well known, is not a 

 symmetrical animal. It has been described as " slipper- 

 shaped " — not a very good comparison, but it will serve 

 lo bring out the fact that the proportion of length and 

 breadth presented to the observer will vary according to 

 the aspect from which the individual is viewed. At what- 

 ever stage of the proceedings the Paramecia took up the 

 position on the slide in which they were measured, they 

 ^^ust have sunk through a layer of fluid the depth of 

 ■vhich was small, no doubt, but considerable in relation 

 -c their size. The conjugant pairs being attached mouth 



NO. T928, VOL. 74] 



to mouth would tend to settle on the broad base presented 

 by the sides of the attached pair, so that one side of each 

 rested on the slide while the other side would be directed 

 to the observer. The non-conjugants might settle on any 

 lateral aspect. Hence a larger proportion of conjugants 

 would be measured in side view than of non-conjugants. 

 This would be another source of error. 



To illustrate the next point I shall refer to another 

 ciliatc infusorian, allied to Paramecium, Leucophrys 

 patula, to which I shall have to return later. It also 

 was investigated by .Maupas (ibid., ser. 2, T. vi., p. 237, 

 and T. vii., p. 250). The ordinary individuals of this 

 species were found to vary in length' from 80 fi to 150 /i. 

 They have a wide ttsophageal recess bordered by vibratile 

 lips (cp. ser. 2, T. vi., Plate xii., Figs. 1-8). The 

 formation of the conjugants occurs by a series of 

 divisions, with progressive reduction in size, of an 

 ordinary individual and of the resulting fission products, 

 giving rise to from eight to thirty-two little conjugants 

 50 ^ to 60 /I in length, and so unlike the non-conjugant 

 form that unless their mode of origin had been ascer- 

 tained, Maupas says, they might be referred to a distinct 

 genus. There are neither vibratile lips nor oesophageal 

 recess, the inouth is closed, and their movements are much 

 more active. Here, then, is a still more marked case of 

 differentiation of gametes than that presented bv Para- 

 mecium caudatum. 



Now the non-conjugant population of the latter species 

 measured by Dr. Pearl to ascertain the range of their 

 variability would include, not only ordinary individuals, 

 but all stages of individuals in process of differentiation 

 as gametes. The non-conjugants are a heterogeneous 

 population ; the conjugants are, on the other hand, approxi- 

 mately homogeneous. This appears to me another and 

 grave source of error in his results on the degree of 

 differentiation and variabilitv of the conjugants. 



Hence, though I am far from denying that it may be 

 true, it appears to me that Dr. Pearl's conclusion is beset 

 with several .sources of error when he atteinpts to give a 

 measure of the degree to which (p. 379) " conjugant 

 individuals when compared with non-conjugants are found 

 to be . . . less variable in both length and breadth." 



I desire to do Dr. Pearl all the justice I can, and his 

 case for homogamy in the conjugation of the gametes 

 appears to me to rest on a sounder basis and to be of 

 interest, though I am doubtful as to the validity of the 

 explanation which he offers for this phenomenon ; but that 

 there is any analogv between it and assortative mating in 

 man, as Dr. Pearl and Prof. Pearson conclude, seeins to 

 me problematical in the extreme. The phenomenon in 

 man which is comparable with the conjugation of the 

 differentiated gametes of Paramecium is the union of the 

 differentiated gametes of man, and I am not aware that 

 it has been shown that there is any correlation between 

 their external characters and the external characters of 

 the human adult. 



Similarly, the conclusion contained in Dr. Pearl's ninth 

 and last heading appears to me altogether unsound. He 

 says (p. 383), speaking of the differentiation of con- 

 jugants, " if the individual Paramjecia of a given race 

 must conform to a definite and relatively fixed morpho- 

 logical tvpe everv tiine they conjugate, what they may 

 acquire during fission generations is clearly of no par- 

 ticular account to the evolutionary history of the race in 

 the long run." This is to ignore the conclusion to which 

 Dr. Pearl's results point (though it had already been 

 established by Maupas and others), that the conjugants are 

 differentiated gametes. It is the nature of a gamete that 

 it is able to transmit the characters of the organism from 

 which it springs, although itself of a size and bodily shape 

 wholly different from that organism. .\re the gametes of 

 Leucophrys patula, though unlike the ordinary individuals 

 in size and other characters noted above, unable to give 

 rise to like forms? .As a matter of fact, if proof were 

 needed, Maupas watched them in process of differentiation 

 into ordinarv individuals. 



In mv address at York I urged biometricians to make 

 sure that the problems thev seek to elucidate are sound 

 from the biological point of view. When asked by Prof. 

 Pearson for an instance of failure in this respect I gave 



