HEREDITY 



generation to generation, and it is 

 useful to distinguish between the 

 persistence of the general funda- 

 mental characters, from which 

 there is never much divergence, 

 and the persistence of more super- 

 ficial and recent features, which is 

 less secure. It is rare for a child to 

 be born whose characters are in 

 any respect at a level below that of 

 the type of modern man ; but it 

 often happens that an individual 

 peculiarity inborn in the parent 

 fails to reappear in the offspring. 

 Inborn Peculiarities 



On the other hand, relatively 

 unimportant peculiarities, such as 

 having only two joints in the 

 fingers (brachydactylia), have been 

 known to persist for six genera- 

 tions, and may in domesticated 

 animals or cultivated plants be- 

 come permanently characteristic 

 of a breed or variety, as in lop- 

 eared rabbits, or in greater cel- 

 andine (Chelidonium), with cut-up 

 leaves. All sorts of inborn peculiar- 

 ities, except of course complete 

 sterility, may be transmitted, 

 whether structural or functional, 

 trivial or important, beneficial or 

 injurious, bodily or mental. Even 

 subtle characters like longevity, 

 fertility, immunity, and suscepti- 

 bility to certain diseases may be 

 transmitted. The word " may " 

 cannot, however, be changed into 

 " must," for against the fact of 

 hereditary persistence or inertia 

 has to be placed the fact of varia- 

 bility or divergence. Like only tends 

 to beget like ; the entail is very 

 frequently broken ; novelties are 

 continually emerging. 



In this connexion it is important 

 to understand that an inborn de- 

 fect in some vital process, such as 

 excretion or digestion, or in some 

 important part of the body such as 

 the brain or the blood, is quite 

 likely to appear in the next gen- 

 eration ; and there is a grievous 

 list showing the heritability of this 

 sort of disease and defect. On the 

 other hand, it is misleading to 

 speak of the trans missJbility of any 

 disease that is due to a microbe. 

 There may be infection before 

 birth, as in the case of syphilis ; or 

 there may be an inheritance of a 

 predisposition to the disease, e.g. a 

 susceptibility to tubercular infec- 

 tion ; or in mammals there may be a 

 handicapping of the offspring be- 

 cause of disease in the mother ; but 

 there is no warrant for speaking of 

 the inheritance of microbic dis- 

 eases as such. 



Some light is thrown on the per- 

 sistence of hereditary characters 

 by the fact of germinal continuity. 

 It was pointed out "by Galton, 

 Weismann, and others that when 

 a fertilised egg-cell is dividing and 



3954 



redividing to form a mass of em- 

 bryonic cells, out of which tissues 

 and organs are soon formed, some 

 of the original germinal material 

 is kept apart, not sharing in body- 

 making, to form the germ-cells of 

 the offspring. In a large number 

 of types this early segregation of 

 definite germ -cells has been de- 

 monstrated ; in other cases all that 

 can be said is that there is a line- 

 age of unspecialised elements which 

 at last leads to the establishment 

 of the essential reproductive organs. 



The general idea is that the 

 characteristic protoplasmic or- 

 ganization (including the associ- 

 ated possibilities or actualities of 

 chemical and other activities) is 

 continued intact along a lineage of 

 non-specialised cells, which eventu- 

 ally lead to the germ-cells of the 

 offspring. The egg liberated by the 

 offspring develops into a similar 

 creature, because it is organically 

 continuous with the fertilised ovum 

 from which the offspring de- 

 veloped. Thus it is not accurate to 

 think of a parent handing on 

 characters to the offspring. It is 

 rather that the offspring inherits 

 from the endless chain of un- 

 specialised germ-cells. For this 

 reason, like tends to beget like. 

 Acquired Characters 



Cases where a new generation, 

 e.g. of potatoes or sponges, is ob- 

 tained by cutting off pieces of the 

 parent's body and planting them 

 out, do not offer any objection. 

 It is probable that all the funda- 

 mental qualities of the fertilised 

 ovum are distributed among the 

 cells of the offspring, where most 

 of them lie latent, except under 

 exceptional stimulation. Body- 

 cells may in rare cases give rise to 

 germ -cells, as in cuttings and 

 liberated buds, but in the animal 

 world the usual method is to have 

 a lineage of unspecialised cells. 



There has been much discussion 

 about the question of the transmissi- 

 bility of individually acquired 

 bodily modifications directly due to 

 peculiarities in function or environ- 

 ment. These "acquired characters," 

 as they are wrongly called, are in- 

 dents from without, and not to 

 be confused with variations or 

 mutations which are outcrops 

 from within. (See Evolution.) 

 Modifications may be illustrated by 

 the permanent browning of a white 

 man's skin by many years of ex- 

 posure to the tropical sun, by the 

 great strengthening of a muscle by 

 persistent exercise, or by a callos- 

 ity developed on the skin as the 

 result of prolonged pressure, and so 

 on. The question is whether these 

 modifications can be handed on 

 as such or even in any degree, so 

 that they reappear by inheritance 



HEREDITY 



in offspring who were not sub- 

 jected to the unusual influence. 



There have been only a few im- 

 portant experiments bearing on 

 the problem, but the answer of the 

 majority of naturalists is that there 

 is not at present any convincing 

 evidence of such transmission. The 

 offspring may re-acquire the par- 

 ental modification if subjected to 

 similar influences ; secondary effects 

 of the parental modification may 

 influence the developing offspring, 

 especially in mammals ; a deeply 

 saturating influence may affect not 

 only the parent's body, but the 

 germ-cells at the same time, so 

 that the offspring come to be 

 altered, though not in the same 

 way as the parents. 



Moreover, in a few cases it has 

 been made probable that a modifi- 

 cation of the parent may produce 

 a chemical substance which gets 

 access to the general cell-substance 

 of the germ-cells, or to the de- 

 veloping embryo or seed, so that 

 there results, as long as the sub- 

 stance lasts, a transient modifi- 

 cation of the offspring similar to 

 that of the parent. But at present 

 the evidence in favour of the trans - 

 missibility of individually acquired 

 modifications even in a faint degree 

 is very far from convincing. The 

 practical corollary as regards man- 

 kind is that increased care should 

 be taken to shield the members of 

 the next generation from influences 

 which are known to produce in- 

 jurious modifications, and to 

 secure for them a persistence of 

 those peculiarities of nurture 

 which wrought out beneficial modi- 

 fications in the parents. 



In 1865 Mendel stated an im- 

 portant law of heredity, which re- 

 mained almost unnoticed till 1900, 

 when De Vries, Correns, and Tscher- 

 mak independently reached ex- 

 perimental results closely resem- 

 bling Mendel's. 



In illustration of characters 

 which exhibit Mendelian inherit- 

 ance, the following may be cited, 

 the dominant character being 

 named first in each case : Horn- 

 lessness and the presence of horns 

 in cattle; normal hair and long 

 " Angora " hair in rabbits and 

 guinea-pigs; crest in poultry and 

 absence of crest; extra toes in 

 poultry and normal four toes ; un- 

 handed shell in wood-snail and 

 banded shell ; yellow cotyledons 

 in peas and green ones ; " round 

 seeds in peas and wrinkled ones; 

 absence of awn in wheat and its 

 presence ; susceptibility to rust in 

 wheat and immunity from this dis- 

 ease ; two-rowed ears of barley and 

 six-rowed ears. 



The Mendelian law of alternative 

 inheritance implies three main 





