1896.] On Heredity and Rejuvenation. 97 



The consideration of the role of rejuvenation in the origin 

 of organs leads us to the theory of Post-selection (Nach- 

 auslese). The theory is by no means new, but I wish to em- 

 phasize its far reaching importance. The preceding discus- 

 sion teaches us to divide the origin of a new morphological 

 part into two stages. The first stage is the development of 

 the rudiment (milage) by multiplication of the cells. The 

 second stage is characterized by the gradual differentiation of 

 the cells, by which they become capable of their ultimate 

 functions. Especially in embryos is the difference in time 

 very marked between the formation and the differentiation of 

 the " Anlagt." Now it is evident that the undifferentiated 

 *' Anlage " is not useful, but becomes useful later. The forma- 

 tion and conservation of the " Anlage," therefore, are due to se- 

 lection, working, not directly upon the " Anlage,'' but indi- 

 rectly through preservation of the fully developed organ- The 

 conception advanced is very simple and appears to me a nec- 

 essary consequence of our knowledge- For the conception 

 itself there has been hitherto to no definite term, I propose, 

 therefore, to call it " Post-selection " (in German, " Kachaus- 

 lese). To avoid possible misunderstanding, I give another ex- 

 ample of post-selection. A parasitic wasp lays its egg in a 

 certain caterpillar; the mother wasp gains no advantage, nat- 

 ural selection does not touch her, but only her progeny, the 

 wasp larva. Nevertheless, the survival of the fittest rules. 



In conclusion, I should like to direct the reader's attention 

 to a problem which, so far as I am acquainted with the litera- 

 ture of biology, has been left almost unconsidered. This pres- 

 ent translation enables me to insert a qualification of the pre- 

 ceding sentence, which ought to have been inserted in the 

 original article, namely, that the problem has been the sub- 

 ject of important discussions by Hyatt, Cope and a very few 

 others among paleontologists. I am glad to be able to refer 

 to the article by Professor Hyatt, (see Jan. Naturalist) and pre- 

 sents the paleontological theory of the loss of ancestral charac- 

 teristics. The problem above referred to is the problem of lost 

 characteristics, which seems to me o«e of the fundamental 

 problems of the doctrine of evolution, because we cannot un- 



