REFLECTIONS ON HEREDITY. 



depends on the fact that a substance of special molecular composition 

 passes over from one generation to another. 



" This is the ' germ-plasm,' the power of which to develop itself 

 into a perfect organism depends on the extraordinary complication of 

 Kb minutest structure. At every new birth a portion of the specific 

 germ-plasm, which the parent egg-cell contains, is not used up in 

 producing the offspring, but is reserved unchanged to produce the 

 germ cells o-i the following generation. Thus the germ cells so far 

 as regards their essential part, the germ-plasm are not a product 

 of the body itself, but are related to one another in the same way 

 as are a series of generations of unicellular organisms derived from 

 one another by a continuous course of simple division. Thus the 

 question of heredity is reduced to one of growth. A minute portion 

 of the very same germ-plasm from which, rirst the germ cell, and then 

 the whole organism of the parent, were developed, becomes the start- 

 ing point of the growth of the child or the calf or other animals. 



" But if this were all, the offspring would reproduce the parent 

 exactly, in every detail of form and structure ; and here we see the 

 importance of sex, for each new germ grows out of the united germ- 

 plasm of two parents, whence arises a mingling of their characters 

 in the offspring. This occurs in each generation, hence every individual 

 is a complex result reproducing in ever-varying degrees the diverse 

 characteristics of his two parents, four grandparents, and other more 

 remote ancestors ; and that ever-present individual variation arises 

 which furnishes the material for natural selection to act upon. 

 Diversity of sex becomes, therefore, of primary importance as the 

 cause of variation. Where a sexual generation prevails the charac- 

 teristics of the individual alone are reproduced, and there are thus 

 no means of effecting the change of form or structure required by 

 changed conditions of existence. Under such changed conditions a 

 complex organism, if only sexually propagated, would become extinct. 

 But when a complex organism is sexually propagated there is an ever- 

 present cause of change which, though slight in any one generation, is 

 cumulative, and under the influence of selection is sufficient to keep 

 up the harmony between the organism anad its slowly changing en- 

 vironment. 



"Growth," says Letourneau, "is only an excess of nutrition, and 

 generation is only an excess of growth. Growth and generation have 

 for cause a sunerabundance of nutritive materials. The superabund- 

 ance has for effect first of all to carry the anatomical elements to their 

 maximum volume, then to provoke the formation of new elements. As 

 long as the animal has not attained all the development compatible 

 with the plan of its being, the elements, newly born, remain aggregated 

 to the pre-existent elements. When the limit of growth is attained, 

 when there is no longer room in the organised animal for a new 

 adjunction of histological elements, the newcomers detach themselves 

 from their organic stem and constitute independent individuals which 

 evolve in their turn. Generation is so much a continuous growth that 

 its processes are identical with or analagous to those of growth." 



By variation is understood the difference between animals not only 

 of the same family but of the same species. Everyone knows that 

 in each litter of puppies no two are alike ; no two human beings are 

 alike ; no two atoms are alike ; which goes to show that there is a 

 higher power than Nature itself. As Wallace says : " This individual 

 variability exists among all creatures whatever, which we can closely 

 observe, even when two parents are very much alike and have been 

 matched in order to preserve some special breed." The same thing 

 occurs in the vegetable kingdom. All plants raised from seed differ 

 from each other. Another very common error is that variation is the 



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