VI.J THEIR SIGNIFICANCE IN HEREDITY. 391 



the ^gg are very different. I might lierc mention various otlier 

 considerations ; but this would lead me too far from my sub- 

 ject, into new theories of heredity. I hope to be able at some 

 later period to develope further the theoretical ideas which arc 

 merely indicated in the present essay. I only wish to show 

 that the consequences which follow from my theory upon the 

 second division of the egg-nucleus, and the formation of the 

 second polar body, are by no means opposed to the facts of 

 heredity, and even explain them better than has hitherto been 

 possible. 



The fact that the children of the same parents arc never 

 entirely identical could hitherto onl}- be rendered intelligible 

 by the vague suggestion that the hereditary tendencies of the 

 grandfather predominate in one, and those of the grandmother 

 in another, while the tendencies of the great-grandfather pre- 

 dominate in a third, and so on. Any further explanation as to 

 why this should happen was entirely wanting. Others even 

 looked for an explanation to the different influences of nutrition, 

 to which it is perfectly true that the egg is subjected in the 

 ovary during its later development, according to its position 

 and immediate surroundings. I had mj^self referred to these 

 influences as a partial explanation ', before I recognized clearlj' 

 how extremely feeble and powerless are the influences of 

 nourishment, as compared w4th hereditary tendencies. Accord- 

 ing to my theory, the differences between the children of the 

 same parents become intelligible in a simple manner from the 

 fact that each maternal germ-cell (I shall speak of the paternal 

 germ-cells later on) contains a peculiar combination of ancestral 

 germ-plasms, and thus also a peculiar combination of hereditary 

 tendencies. These latter by their co-operation also produce a dif- 

 ferent result in each case, viz. the offspring, which arc charac- 

 terized by more or less pronounced individual peculiarities. 



But the theory which explains individual differences by 

 referring to the inequality of germ-cells, may be proved with a 

 high degree of probability by an appeal to facts of an opposite 

 kind, viz. by showing that identity between offspring only 

 occurs when they have arisen from the same cgg-ccll. It is 



1 Weismann, ' Studicn ziir Descendcnzthcoric,' ii. p. 306, Leipzip, 

 1876, translated by Mcldola; sec 'Studies in the Theory of Descent,' 

 p. 680. 



