September 29, 192 1] 



NATURE 



145 



This vision is not new, and, human nature being 

 what it is, a vision it is likely to remain. Still, if 

 science is ••organised common sense," something may 

 be done bv applving it to the machinery of publication. 

 Dr. Brieriey has made a beginning by taking a square 

 look at the subject. J- A. H. 



Jermyn Street, London, S.W., September 19. 



Is Bisexuality in Animals a Function of Motion? 



It was long ago pointed out by Claus that her- 

 maphroditism in animals finds its most frequent ex- 

 pression in fixed and sluggish animals, and in certain 

 parasites. A large amount of work on sex-phenomena 

 has been done since Claus 's time, and his generalisa- 

 tion has been confirmed. There is thus no doubt 

 of the fact that there is a close correlation between an 

 inactive life and hermaphroditism, so close indeed that 

 the present writer is ready to cast suspicion on the 

 supposed bisexuality of any sedentary animal. It is 

 seen therefore that absence of motion is in some way 

 directly or indirectly associated with an hermaphrodite 

 expression of sex. 



After Claus a number of writers suggested that fixed 

 or sluggish animals or parasites would accumulate an 

 excess of nutritive products, a condition which was 

 considered sufficient in itself to account somehow for 

 hermaphroditism. This point of view was accepted 

 and developed by Geddes and Thompson, who arrived 

 at the conclusion, in their work on " Evolution and 

 Sex," that " hermaphroditism is primitive and that 

 the unisexual state is a subsequent differentiation : the 

 present {exxslxng) ^ cases of normal hermaphroditism 

 (therefore) imply either persistence or reversion." 

 More recent workers appear to have neglected the 

 wider aspects of the problem of sex, and to have con- 

 centrated on the question of the mechanism of sex- 

 determination. Recent discoveries of hermaphrodit- 

 ism, and particularly some cases studied by the writer, 

 have directed attention to a speculation which indeed 

 follows naturally from Claus 's observations, namely, 

 is hermaphroditism a direct physical consequence of a 

 fixed or sluggish life, and conversely, is bisexuality in 

 a species a direct physical consequence of a freely 

 moving habit of life? 



This view has been examined in a preliminary way 

 and has been tested in some of the present writer's 

 recent researches with results which are meagre, but 

 promise more success by a method which involves 

 following closelv the life-histor\- of individuals of a 

 species. It is to be expected, moreover, that with all 

 the ups and downs in the progress of evolution there 

 will exist an infinite variety of circumstances to obscure 

 such an underlying principle as that suggested above, 

 even should the principle obtain in nature. A phylo- 

 genetic explanation of hermaphroditism is unsatisfac- 

 tory- in that it is inherently inconclusive, and in that 

 there is no method of testing it, whereas the specula- 

 tion advanced here can be tested, and, if found want- 

 ing, can be rejected. 



In a sur\'ey of the sex-phenomena in the animal 

 kingdom it is found, generally speaking, that forms 

 which are slow-moving, sedentan.-, or of fixed para- 

 sitic habit are hermaphrodite — i.e. an individual will 

 produce eggs and sperm either simultaneously or sepa- 

 rately in the course of its life-history — and that 

 animals which are active during life are bisexual, 

 whilst animals which fall into an arbitrarv category 

 of sluggish or slow-moving forms, for example Lam- 

 ellibranch Molluscs, exhibit a wide range of sex-phe- 

 nomena, from apparent bisexuality to definite simul- 

 taneous hermaphroditism. This is not the place to go 



1 The words in italics have be«n inserted by the present writer. 



NO. 2709, VOL. 108] 



into full technical details, but it may be mentioned 

 that in the category of fixed or sedentary or sluggish 

 animals (excluding Protozoa) which are hermaphrodite 

 may be entered : Sponges, so far as sex is known in 

 the group, sedentary Coelenterates (but apparently 

 not all), practically all the members of the follow- 

 ing groups — Platyhelminthes (the flat-worm group), 

 Leeches, Oligochaeta, Polyzoa, Ascidians, euthy- 

 neurous and pteropodous Molluscs, practically all 

 fi.xed Arthropods, i.e. Cirripedia — and various seden- 

 tar\- forms in other groups. Amongst animals which 

 are free-living, active, and bisexual are most Arthro- 

 pods, all cephalopodous and most streptoneurous Mol- 

 luscs, and all vertebrates except a few sluggish or 

 parasitic forms. In the intermediate category of 

 groups which are composed mainly of slow-moving 

 forms mav be placed Lamellibranch Molluscs, Amphi- 

 neura, Echinoderms, Polychaetes, and Nemertine 

 worms. In all these latter groups hermaphroditism 

 occurs to some extent," and indeed Mortensen ^ has 

 shown recently that a very large number of Ophiuroids 

 are hermaphrodite. Now, in view of recent work on 

 hermaphroditism there is legitimate reason to doubt 

 the supposed bisexuality in a very large number of 

 forms belonging to the groups mentioned here, as 

 may be seen from a review of the sex-phenomena 

 in the oyster and the common limpet. In the 

 oyster (O. edulis) we now know that some forms 

 change from males into females and back again into 

 the male condition. In this species, therefore, looked 

 at as a whole, males change into females and females 

 into males. In the limpet (Patella vulgata) it would 

 appear that some males change into females, but that 

 individuals very rarely show signs of sex-change owing 

 to a distinct break between the male-functioning stage 

 and the female-functioning stage. It is therefore 

 possible that in some supposed bisexual species males 

 may change into females and females change into 

 males, since a parallel series of changes is known to 

 occur in the oyster * (O. edulis). In such cases the 

 actual sex-conditions in the species could only be 

 found out by following the life-history of individual 

 animals. 



If it be supposed that hermaphroditism is a direct 

 physical result of a sedentary life, and bisexuality a 

 direct physical result of an active life, it would appear 

 that there should be some fundamental difference in the 

 physical condition of the general body or in particular 

 organs — e.g. the gonad — in these two classes of 

 animals. This deduction should therefore yield a 

 method of testing the speculation. I do not know of 

 any such fundamental difference in physical condition 

 between sedentar\- and active forms, but the known 

 electrical changes which occur during the activation of 

 muscular and ner\ous tissues, together with the results 

 of recent studies on electrical characters of tissue ele- 

 ments, lead one to hope that if physical differences of 

 this kind do occur they may be found in the near 

 future. .\ fundamental difference in physical condi- 

 tion between sedentar\- and active animals, correlated 

 with different sexual conditions, is probably the touch- 

 stone of this speculation, and offers a definite problem 

 for bio-physicists. Contemplation of success in the 

 discover}- of such a fundamental difference arouses 

 visions of the determination of sexual characters of 

 species by some sort of physical constant, and a faint 



* I have found that Lineus lacteus — a tlueei^h Xeraertine living in gravel 

 at half-iide level — is definitely hermaphrodite ; both eggs and sperm are 

 mature in a single individual at the same time. 



3 Th. Mortetisen, " On Hermaphroditism in Viviparous Ophiuroids," 

 Acta Zoologica, pp. 1-18, 1920. (Stockholm.) 



* It must be pointed out that we have as yet no dear proof that all male 

 oysters or aJ/ male common limpets change into female?; an i the pre'sent 

 writer's work has shown th«t sexchanee in the slipper-limpet from male to 

 female may be almo't indefinitely delayrd accordins: to circumstances 

 (Proc. Roy. Soc., B, vol. 3i, 1909, pp. 468-S4, and unpublished workX 



