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individuals which presented them largely developed, or possessed a higher degree of plasticity in those parts. Lowly organized structures retain much plasticity from their tendency to vary never having been checked by selection. Such forms are probably never as variable as those which are undergoing rapid modification; but it is extremely difficult to judge of the amount of variability in very different forms of life. Where few characters exist, the variability of one is liable to be regarded as of undue importance, when compared with the variation of a single character in organisms presenting a great number of special organs and highly differentiated parts.
Of all organized beings, man is probably the most easily influenced and modified by the action of external and internal causes. The human 'species, and especially the higher races of man, are the most plastic of all animals. No creature probably presents such strongly marked individual differences. The races of mankind are most distinct; and man is able not only to accommodate his organism to a great range of conditions, but is likewise endowed with extreme delicacy of constitution, at least in his most highly developed or civilized form; and diseases of far more manifold types are the result, as well as far more frequent and serious deviations of development, even when
compared with the more plastic domestic races of animals.
This great plasticity is probably due to the rapid stages by which his organization has progressed, especially amongst civilized races, where those individuals whose intellectual faculties vary most in the direction of acuteness and quickness of action are greatly favoured in the struggle for life : hence, probably, that undue plasticity of the brain by which insanity is ever on the increase amongst highly intellectual races. No man versed in history and mental disease can for a moment doubt the insanity of such men as Caligula, Nero, Domitian, and Commodus, or the fact that they were the descendants of the greatest families of a race which had culminated with excessive rapidity.
It would therefore appear that rapid modification, by the selection of the most variable and therefore the most plastic forms, increases plasticity, and that want of variation in the conditions of life tends towards the establishment of rigidity as soon as the organism becomes adapted to the conditions around it. Hence changed conditions are of the highest importance in causing variability.
Two other conditions are believed to affect variability to a great degree, excess of food and long-continued transmission. Andrew Knight be
lieves that variability is in some way connected with excess of food; and the opinion is held pertinaciously by most breeders, that a character becomes fixed by long transmission: both these opinions, however, will be more conveniently discussed in the next chapter.
It is no part of the present plan to prove the great generality of the transmission of characters from the parent to its offspring. In all ordinary cases it is a matter of common observation that each kind produces young like itself; amongst breeders it is a deep ingrained belief that all characters, both normal and abnormal, are inherited; and Mr. Darwin, whose minute acquaintance with the subject entitles him more fully than any other to express an opinion, says :-"Perhaps the correct way of viewing the whole subject would be to look at the inheritance of every character as the rule, and its non-inheritance as the anomaly ” *.
Inheritance is apparently such a mysterious power, that it requires no slight mental effort to view it as a mere expression of the fact that all variation is limited.
Before entering upon the consideration of inheritance viewed in this light, it will be necessary to attain some definite ideas of the relation subsisting 50
* Origin of Species, p. 10.
RELATION OF PARENT AND OFFSPRING.
between the parent and its offspring. As this subject is one of great complexity, and as it will be considered at some length in another chapter, it will be sufficient for the present to regard the offspring as a portion separated or broken off from the parent. In the simplest forms of life, as in the Amoeba and Rhizopoda generally, this is strictly true, the parent mass merely divides into separate portions as it increases in size; and in cases of reproduction by gemmation or budding the same relation subsists. In sexual reproduction the case is far more complex ; but there is abundant evidence * that the transition from buds to ova is very gradual ; and that from agamo- to gamogenesis is far from abrupt. The great difficulty lies in the comprehension of the process of development, rather than in that of hereditary transmission ; and the consideration of this subject will be entered into in the next chapter.
We have already seen that the plasticity, and therefore the variability, of an organism may be greatly modified by natural selection. It must not be supposed, however, that all inheritance is ascribed here to such loss of plasticity ; great deviations from any acquired form and structure are probably prevented by far more definite causes ;
* Darwin's - Animals and Plants under Domestication,' vol. č. p. 358.