HEREDITY 



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HEREDITY 



influenced by peculiarities in the 

 nurture which is available for it. 

 The same is true in all cases where 

 the parents, plants as well as 

 animals, nurture the offspring. 



An important peculiarity in the 

 nurture, whether favourable or 

 prejudicial, may produce a change 

 or modification in the offspring. 

 This change is not part of the 

 hereditary nature ; it is conveni- 

 ently referred to as a result of some 

 peculiarity of nurture. The natural 

 inheritance includes all that the 

 creature is or has to start with, 

 when it is in the germ-cell stage of 

 its being, in virtue of its relation of 

 organic continuity with the ger- 

 minal material of its parents and 

 ancestry. 



Development is the realization of 

 the natural inheritance, the making 

 actual that which lies invisible and 

 latent in the fertilised egg-cell. In 

 figurative language development 

 may be called cashing the inherited 

 legacy and trading with it. Normal 

 development implies appropriate 

 nurture, a succession of chemical 

 and physical stimuli due to food, 

 oxygen, humidity, warmth, etc., 

 which serve to evoke the potential- 

 ities of the germ and the embryo. 



Peculiarities in this nurture may 

 divert the development from its 

 normal course, and in a lineage 

 whose natural inheritance is good 

 there may be an outcrop of ab- 

 normalities, because of quite ex- 

 trinsic defects of nurture, such as 

 lack of food or very low tempera- 

 ture. What an organism becomes 

 is primarily dependent on the in- 

 teraction of the hereditary nature 

 and the nurture that is available. 



The natural inheritance has its 

 material basis in the germ-cells 

 the ovum or egg-cell and the sper- 

 matozoon or sperm-cell. Whatever 

 be the precise nature of the germ- 

 cells, they are the exclusive ve- 

 hicles of the inheritance. Although 

 we cannot in any way picture it, 

 the heritable qualities or the or- 

 ganization which makes these 

 qualities possible must be entirely 

 contained within the germ-cells, 

 which are usually microscopic. 

 The Human Germ-cell 



The human ovum is only about 

 0*2 of a millimetre in diameter, and 

 many ova measure not more than a 

 few thousandths of a millimetre. 

 But the germ-cells are not to be 

 thought of as ordinary cells ; they 

 are individualities in a unicellular 

 state, rich in initiatives. The fer- 

 tilised ovum contains in some 

 mysterious implicit state all the 

 potentialities of the organism into 

 which in favourable circumstances 

 it will develop. But it is possible 

 to go further. Inside each germ- 

 cell there is a kernel or nucleus, as 



is usual in cells, and inside the 

 nucleus there is a definite number 

 of readily stainable rodlets or 

 granules, called chromosomes. 

 Many facts point to the conclusion 

 that although the general substance 

 (or cytoplasm) of the germ-cells 

 counts in inheritance, the chromo- 

 somes are the special bearers of the 

 factors or determiners of the 

 hereditary qualities, or, at least, 

 of many of them. 



Except in cases of virgin birth 

 or parthenogenesis, where ova de- 

 velop without fertilisation, as in 

 summer green-flies, and many 

 wheel-animalcules, the inheritance 

 must be dual. It is borne by a 

 spermatozoon contributed by the 

 male parent and by an ovum con- 

 tributed by the female parent, and 

 these two cells unite in an in- 

 timate and orderly way to form the 

 fertilised ovum. There are, indeed, 

 many cases where the parents have 

 both egg-producing and sperm- 

 producing organs ovaries and 

 testes thus common animals like 

 snails, earthworms, and leeches 

 are hermaphrodite but even then 

 cross-fertilisation is the rule and 

 the inheritance is dual. 



Duality of Inheritance 



Very rarely does an animal fer- 

 tilise its own eggs ; this autogamy, 

 as it is called, is illustrated by 

 some tapeworms and flukes. In 

 the vast majority of cases the 

 inheritance is dual, and the num- 

 ber of chromosomes contained in 

 the two kinds of germ-cells is 

 typically the same. As the egg-cell 

 is usually much larger than the 

 spermatozoon, the maternal parent 

 contributes more of the initial 

 building-material or formative pro- 

 toplasm ; the egg is often provided 

 with a nutritive yolk. 



On the other hand, the sperma- 

 tozoon introduces into the egg-cell 

 a minute body known as the cen- 

 trosome, which plays an important 

 part in the subsequent cleavage or 

 segmentation of the fertilised egg. 

 In recognizing the fact that the in- 

 heritance is typically dual, it must 

 be noted that the paternal and 

 maternal characteristics are not 

 likely to find equal expression in 

 the development of the offspring. 



A distinction must be drawn be- 

 tween the germinal constitution 

 the natural inheritance contained 

 in the fertilised egg-cell and the 

 expression of the inheritance in 

 the development of the offspring. 

 Characters often lie latent for one 

 generation and find expression in 

 the next. Again, while the in- 

 heritance is dual, it is in another 

 aspect multiple, since the offspring 

 may exhibit ancestral characters 

 not expressed in the parents. 



Statistical inquiries as to the in- 



heritance of stature and other 

 qualities in man, and as to coat- 

 colour in Basset hounds, led Sir 

 Francis Galton to his Law of 

 Ancestral Inheritance, according 

 to which the two parents between 

 them contribute, on the average, 

 one-half of each inherited faculty, 

 each of them contributing one- 

 quarter of it ; the four grand- 

 parents contribute between them 

 one-quarter, or each of them one- 

 sixteenth, and so on backwards. 

 Prof. Karl Pearson has altered 

 Galton's series from 0*5, 0'25, 

 0-125, etc., to 0-6244, 0'1988, 

 0-0630, etc., but the general idea 

 remains that the average con- 

 tributions made by grandparents, 

 great-grandparents, etc., diminish 

 in a precise ratio according to the 

 remoteness of the ancestors. 

 Law of Filial Regression 



A correlated generalisation is 

 known as the law of filial regression, 

 which means that the offspring of 

 exceptional parents tend on the 

 average to approximate (up or 

 down) to the mean of the general 

 population. To take Prof. Karl 

 Pearson's instance, with a collec- 

 tion of fathers of stature 72 ins., 

 the mean height of their sons will 

 be 70-8 ; with a collection of 

 fathers of 66 ins., the mean height 

 of their sons will be 68'3 in both 

 cases an approximation towards 

 the mean of the general population. 



These two generalisations are 

 statistical average conclusions, not 

 individual physiological conclu- 

 sions, but they do not appear to 

 apply to sharply defined non-blend- 

 ing characters, which exhibit what is 

 called Mendelian inheritance. Nor 

 is it clear that the data utilised 

 were sufficiently homogeneous, for 

 peculiarities or modifications di- 

 rectly due to peculiarities of in- 

 dividual nurture must not be 

 mixed up with peculiarities due 

 to intrinsic germinal variations. 

 Caution is necessary in dealing with 

 these statistical conclusions. 



The largest fact in regard to 

 heredity, and at the same time its 

 central problem, is the persistence of 

 a particular kind of organization and 

 activity from generation to genera- 

 tion. Grapes are not gathered from 

 thorns or figs from thistles : like be- 

 gets like is confirmed by modern 

 research which has demonstrated 

 what is called the " specificity" or 

 "individuality" of each kind of 

 creature. Each has its own parti- 

 cular kind of living matter, with a 

 definite microscopic and ultra-mi- 

 croscopic architecture, and with a 

 system of chemical reactions differ- 

 ent from those of other creatures. 



The largest fact of inheritance is 

 the more or less perfect maintenance 

 of this sum of characteristics from 



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