678 



HEREDITY 



there stands the great fact of the real continuity 

 of generations. 



The Problem of Reconstruction. How is it that 

 the germ-cell divides and redivides as it does, and 

 how does the development of the emhryo retain its 

 architectural constancy ? Part of the answer has just 

 been given : hecause the germ is virtually continuous 

 with, and made of the same stuff as, the parent 

 germ ; therefore it must behave in precisely similar 

 fashion. The rest of the answer involves difficulties 

 which cannot fairly be laid on the shoulders of 

 students of heredity, but belong to that most 

 intricate of problems, the mechanics of develop- 

 ment. Referring to the article EMBRYOLOGY for 

 notice of some of the pioneer investigators of this 

 problem, we can do little more than reiterate the 

 caution of Professor His : ' To think that ' ' hered- 

 ity " will build up organic beings without mechani- 

 cal means is a piece of unscientific mysticism.' 

 We must also protest against the careless diction 

 which makes ' heredity ' now into a ' principle ' and 

 again into a ' power, which calls it sometimes a 

 ' law ' and next time a ' cause.' 



Inheritance of Acquired Characters. Changes or 

 variations in an organism may be roughly referred 

 to three origins: (a) they may be the results of 

 external or environmental influence; (b) they may 

 be the outcome of use and disuse, or of functional 

 increase or decrease ; or ( c ) they may be due to 

 internal, constitutional, or germinal conditions, of 

 which one of the most important is the mingling of 

 two different kinds of living matter in the fertilis- 

 ation of the egg-cell. It is granted by all that an 

 individual plant or animal may exhibit these three 

 kinds of variation environmental, functional, .and 

 organismal ; and it is also true that the majority of 

 naturalists have till recently believed that an indi- 

 vidual gain or loss from any of the above origins 

 might be transmitted from parent to offspring. 

 No\v, however, there is a widespread scepticism as 

 to the inheritance or transmission of any but 

 organismal, congenital, or germinal ' variations. 

 This scepticism, mainly emphasised by Weismann, 

 and now prevalent among naturalists, is by no 

 means novel. The editor, whoever he was, of 

 Aristotle's Historia Animalium seems to have 

 differed from his master as to the inheritance of 

 injuries and the like. Kant also maintained the 

 non-inheritance of extrinsic variations, and Blumen- 

 bach cautiously inclines to the same negative posi- 

 tion. In more recent times, His expressed a strong 

 conviction against the inheritance of acquired char- 

 acters, and Pfiiiger is also among the sceptics. A 

 few sentences from Galton ( 1875), whose far-sighted- 

 ness has been insufficiently acknowledged, may be 

 quoted. The inheritance of characters acquired 

 during the lifetime of the parents ' includes much 

 questionable evidence, usually difficult of verifica- 

 tion. We might almost reserve our belief that the 

 . structural cells can react on the sexual elements at 

 all, and we may be confident that at the most they 

 do so in a very faint degree in other words, that 

 acquired modifications are barely, if at all, inherited 

 in the correct sense of that word.' Weismann, 

 however, has brought the discussion to a climax. 

 He goes even further than Galton in scepticism as 

 to the inheritance of acquired characters, for he 

 denies that any such transmission occurs. This 

 denial is in part justified by the absence of experi- 

 mental evidence to the contrary, but it is also 

 suggested by Weismann's theory of continuity. 

 For if a portion of the germ -plasma of a fertilised 

 ovum is preserved unchanged during development 

 to form the rudiments of the reproductive cells of 

 the new organism, and if the germ-plasma is as 

 stable as Weismann makes out, then there is a 

 strong probability that no variatiohs produced in 

 the body by use or disuse or by outside influences 



can be transmitted. For they could only be trans- 

 mitted by affecting the germ-cells, and this is a 

 possibility which Weismann denies. He makes, 

 however, two admissions: (a) that the germ-plasma 

 may be slightly modified by changes of nutrition 

 and growth in the body, and ( b ) that external con- 

 ditions such as climate may influence the germ-cells 

 along with, though not exactly through, the body- 

 cells. These admissions are of course different from 

 the once prevalent opinion that changes in the body 

 were able to affect the germ-cells, and thus become 

 transmissible, though it may be questioned whether 

 the two saving clauses which Weismann allows are 

 not sufficient to dainage seriously the stringency of 

 the conclusion on which he insists throughout that 

 no acquired characters are transmissible. 



If this conclusion be true, then the influences of 

 function and environment on the body of an organ- 

 ism affect the individual only, not the species. 

 They liave therefore no evolutionary value ; the 

 source of variation and the origin of adaptations 

 must be sought elsewhere. To Weismann the sole 

 source of evolutionary change is the intermingling 

 of germ-plasma which occurs in fertilisation, and 

 the condition of progress is found in the action of 

 natural selection on the germinal variations which 

 thus arise. There are, however, evolutionists who 

 regard species as the necessary results of persistent 

 variation in some definite direction, ' according to 

 the laws of organic growth,' 'according to the con- 

 ditions of protoplasmic change,' 'according to the 

 opposition between nutrition and reproduction,' and 

 so on. Those who take this view, even if they admit 

 Weismann's conclusion about acquired characters, 

 will not find it necessary to lay the entire burden 

 of progress on the shoulders of natural selection. 



As Weismann's conclusion that acquired char- 

 acters are not transmitted is one of vast importance 

 both theoretically and practically, it is necessary to 

 notice some of the counter arguments. ( a ) There 

 are very numerous cases on record where the effects 

 of mutilation are said to be inherited, but it must 

 frankly be allowed that no case is known which is 

 not open to serious objection. Circumcision has 

 a very ancient origin, but its effects on the Jewish 

 race are imperceptible ; while the same is true of 

 mutilations inflicted for many generations on 

 domesticated animals. And even the numerous 

 cases of tailless kittens pr&duced from artificially 

 curtailed cats have little cogency in face of the fact 

 that tailless sports may also arise from normal 

 parents. (b) Various pathologists have brought 

 forward instances of what appeared to them to be 

 the transmission of acquired disease, but their 

 arguments, as in the case of Virchow's, have evi- 

 denced misunderstanding as to Weismann's real 

 position. There is no doubt that many malforma- 

 tions and weaknesses appear through numerous 

 generations, but there is no evidence that such 

 variations were not to start with germinal. If so, 

 Weismann of course admits their transmissibility. 

 Colour-blindness has been known to occur in the 

 males only of six successive generations, deaf- 

 mutism for three, finger-malformations for six, 

 and so with harelip and cleft-palate, and with 

 tendencies to consumption, cancer, gout, rheuma- 

 tism, bleeding, &c. But none of these prove 

 the transmission of characters acquired by use or 

 disuse, or impressed by the action of surroundings. 

 ( c ) Various naturalists have brought forward what 

 appear to them to be examples of the genuine 

 transmission of individually acquired characters. 

 Thus, Detmer and Hoffmann among botanists, and 

 Eimer among zoologists, may be quoted. Even the 

 title of Eimer 's recent work, The Origin of Species, 

 on the Basis of the Inheritance of acquired Charac- 

 ters, according to the Laws of Organic Growth, 

 shows how far he is from giving up the case. It 



