ZOOLOGY SINCE DARWIN. 485 



On the contrary, Weisniann's neo-Darwinism denies the inheritance 

 of acquired characters, and there exists no well-authenticated fact that 

 contradicts his views nor any theory that can give a plausible explana- 

 tion of the transmission to the organs of the body of new structural 

 elements for the germ cells. 



Should we accept this inheritance of acquired characters, or, what is 

 the same thing, an immediate morphogenic and hereditarily transmis- 

 sible action of external forces, then it would be equivalent to admitting 

 the possibility of the production of new living and fertile animal forms 

 of the most varied kinds by changing the conditions of existence in 

 the eggs of the same species. This is the last consequence, and one 

 contrary to all experience, of the hypothesis that external conditions 

 are the factors that determine form. 



Oscar Hertwig and Y. Delage seek to avoid this conclusion, which 

 they also recognize as absurd, by tracing back the essential similarity 

 of child and parent to the specific chemico-physical constitution of the 

 germ-plasm derived from the latter. 1 The derivation of a species from 

 the germ of another is prevented because every germ must perish that 

 does not find the environment adapted to its specific constitution. 

 But this is only removing the causes of morphogenesis to the intimate 

 constitution of the germ. 



To attribute the same causal significance to these true causes of 

 morphogenesis which lie in the intimate constitution of the germ as to 

 the influence of external conditions is, however, evidently arbitrary, 

 and indicates, as F. v. Wagner has quite recently 2 justly remarked, a 



1 Y. Delage, La stucture da protoplasma et les theories sur l'hereditd et les grands 

 problemes de la biologie generale. Paris, 1895. Delage's views on the heredity 

 question are the exact opposite of those of Weisinann. Characteristic of this is 

 Delage's statement that for the elucidation of the morphological and physiological 

 agreement between parent and child we no more need to accept a tendency to inher- 

 itance residing in the germ than Ave do to explain why the cadaver of an earth- 

 worm, of an insect, of a frog, and of a mammal under the same external conditions 

 all pass in the same way, typical for each animal named, through the processes of 

 decomposition. 



O. Hertwig, Preformation oder Epigenese ? Grundziige einer Entwicklungstheorie 

 der Organismen. Jena, 1894. Hertwig harmonizes the opposing views of Weisinann 

 and Delage, in that he does not assign the causes of the generation of form either 

 exclusively to the germ, nor exclusively or prepouderatingly to the environment, 

 hut gives to each an equal determining value. Hertwig himself says of his theory 

 (pp. 132-133) : "This theory may be called evolutionary, because it accepts for the 

 basis of the processes of development a specific and highly organized primitive sub- 

 stance; it is, on the contrary, epigenetic in so far as it considers that the primitive 

 substance increases only by the fulfillment of innumerable conditions, in which, for 

 example, I include the chemical processes that begin with the first segmentation of 

 the cell, gradually shaping itself, step by step, till it finally produces a maturely 

 developed result as different from its first primitive condition as the completely 

 organized plant or animal is from the single cells which compose it." 



2 F. v. Wagner, Einige Bemerkungen zu O. Hertwig's Entwicklungs-Theorie 

 (Biologisches Centralblatt, XV Bd., pp. 777-815, Leipzig, 1895). The same, Das 

 Problem der Vererbung (''Aula" Wocbenblatt fur die Gebildeten aller Stiinde, I. 

 Jahrg., Nr. 24 und 25, Miinchen, 1895). 



