314 NATURAL SCIENCE. November. 



render plausible the idea that new characters appearing in the organs 

 of the body are transferred to 'the germ-cells. If we accepted this, or 

 — what comes to the same thing — a direct action of the environment 

 producing heritable modifications, we should have to admit that from 

 the eggs of any single species new forms of the most varied descrip- 

 tion, capable of life and reproduction, could be obtained by simply 

 changing the conditions of existence. This — which is contrary to all 

 experience — is the final consequence from the assumption that external 

 conditions are the factors which decide form. O. Hertwig and 

 Y. Delage' seek to escape these conclusions — which they too consider 

 absurd — by attributing the essential similarity of child and parent to 

 the specific chemico-physical composition of the germ-plasm derived 

 from the parent. The production of one species from the germ of 

 another would always be prevented, because every germ must die 

 which does not find the life-conditions adequate to its specific com- 

 position. But this means nothing else than that the causes of form 

 lie in the internal composition of the germ. 



In the face of these true causes of form inherent in the germ 

 itself, to attribute equal causal influence to external conditions is 

 obviously arbitrary, and, moreover, implies — as F. von Wagner^ has 

 shown — a confusion of "cause" and "condition" contrary to the 

 customary use of the ideas. 



The System and the comparative anatomy of organisms show us 

 that a seemingly infinite number of developmental possibilities lie open 

 to the germs, and therefore if certain germs always pursue the same 

 path as did their countless generations of ancestors, this can be due 

 only to internal causes ; the path, however, in order to be followed, 

 must lie open — that is, there must exist certain external circumstances 

 ("conditions"), in order that the causes may result in visible action. 



^ Y. Delage, " La Structure du Protoplasma et les theories sur I'heredite et les 

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

 are the exact antithesis to Weismann's. Characteristic is his contention that in 

 order to explain the morphological and physiological agreement between parent and 

 child, the idea of a hereditary substance inherent in the germ is no more required 

 than it is to make us understand why the corpse of a worm, of an insect, of a frog 

 or of a mammal, under the same external conditions, always undergoes decomposi- 

 tion in its own peculiar way, each creature having its own typical process. 



O. Hertwig, " Praformation oder Epigenese? Grundziige einer Entwicklungs- 

 theorie der Organismen." Jena, 1894. English translation by P. Chalmers Mitchell, 

 reviewed in Natural Science, vol. ix., p. 270. Hertwig stands midway between 

 Delage and Weismann, inasmuch as he does not put the causes of form exclusively 

 in the germ, or exclusively or principally in external conditions, but thinks them 

 both equal in their influence. Hertwig says himself of his theory (pp. 132-133) : — 

 " My theory may be called evolutionary, because it assumes the existence of a 

 specific and highly-organised initial plasm as the basis of the process of develop- 

 ment. It may be called epigenetic, because the rudiments grow and become 

 elaborated, from stage to stage, only in the presence of numerous external conditions 

 and stimuli, beginning with the metabolic processes preceding the first cleavage of 

 the egg-cell, until the final product of the development is as different from the first 

 rudiment as adult animals and plants differ from their constituent cells." 



2F. v. Wagner, "Some Remarks on O. Hertwig's Entwickelungs-Theorie " 

 (Biologisches Centralblatt, xv. Bd., pp. 777-815, Leipzig, 1895), and "Das Problem der 

 Vererbung" {Aula, i. Jahrg., nos. 24 and 25, Munich, 1895). 



