780 



SCIENCE. 



[N. S. Vol. XX. No. 519. 



enon of the semi-permeability of living 

 mattei". 



IV. 



DEVELOPMENT AND HEREDITY. 



We now come to the discussion of those 

 phenomena which constitute the specific 

 difference between living machines and the 

 machines which we have thus far been able 

 to make artificially. Living organisms 

 show the phenomena of development. 

 During the last century it was ascertained 

 that the development of an animal egg, in 

 general, does not occur until a sperma- 

 tozoon has entered it, but, as already 

 stated, we do not know which variable in 

 the egg is changed by the spermatozoon. 

 An attempt has been made to fill the gap 

 by causing unfertilized eggs to develop 

 with the aid of physicochemical means. 

 The decisive variable by which siich an 

 artificial parthenogenesis can be best pro- 

 duced is the osmotic pi*essure. It has been 

 possible to cause the unfertilized eggs of 

 echinoderms, annelids and mollusks to de- 

 velop into swimming larvae by increasing 

 transitorily the osmotic pressure of the 

 surrounding solution. Even in vertebrates 

 (the frog and petromyzon), Bataillon has 

 succeeded in calling forth the first proc- 

 esses of development in this way. In other 

 forms specific chemical influences cause the 

 development, e. g., in the eggs of star-fish 

 diluted acids and, best of all, as Delage has 

 shown, carbon dioxide. In the eggs of 

 Chceto'topterus potassium salts produce this 

 result and in the case of Amphitrite, cal- 

 cium salts. 



From a sexual cell only a definite organ- 

 ism can arise whose properties can be pre- 

 dicted if we know from which organism the 

 sexual cell originates. The foundations 

 of the theory of heredity were laid by 

 Gregory Mendel in his treatise on the 

 ' Hybrids of Plants, ' one of the most prom- 

 inent papers ever published in biology. 

 Mendel showed in his experiments that 



certain simple characteristics, as, for ex- 

 ample, the round or angular shape of the 

 seeds of peas or the color of their endo- 

 sperm is already determined in the germ 

 by definite determinants. He showed, 

 moreover, that in the case of the hybridiza- 

 tion of certain forms one half of the sexual 

 cells of each child contains the determin- 

 ants of the one parent, the other half eon- 

 tains the determinants of the other parent. 

 In thus showing that the results of hybrid- 

 ization can be predicted numerically not 

 only for one but for a series of generations, 

 according to the laws of the calculus of 

 probability, he gave not a hypothesis, but 

 an exact theory of heredity. Mendel's ex- 

 periments remained unnoticed until Hugo 

 de Vries discovered the same facts anew, 

 and at the same time became aware of 

 Mendel's treatise. 



The theory of heredity of Mendel and de 

 Vries is in full harmony with the idea of 

 evolution. The modern idea of evolution 

 originated, as is Avell known, with Lamarck, 

 and it is the great merit of Darwin to have 

 revived this idea. It is, however, remark- 

 able that none of the Darwinian authors 

 seemed to consider it necessary that the 

 transformation of species should be the ob- 

 ject of direct observation. It is generally 

 understood in the natural sciences either 

 that direct observation should form the 

 foundation of our conclusions or mathe- 

 matical laws which are derived from direct 

 observations. This rule was evidently con- 

 sidered superfluous by those writing on 

 the hypothesis of evolution. Their scien- 

 tific conscience was- quieted by the assump- 

 tion that processes like that of evolution 

 could not be directly observed, as they oc- 

 curred too slowly, and that for this reason 

 indirect observations mi;st suffice. I be- 

 lieve that this lack of direct observation 

 explains the polemical character of this 

 literature, for wherever we can base 

 our conclusions upon direct observations 



