RECENT DEVELOPMENT OF BIOLOGY 17 



From a sexual cell only a definite organism can arise, whose pro- 

 perties can be predicted 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 prominent papers ever published in biology. Mendel 

 showed in his experiments that certain simple characteristics, as, 

 for example, the round or angular shape of the seeds of peas or the 

 color of their endosperm, is already determined in the germ by definite 

 determinants. He showed, moreover, that in the case of the hy- 

 bridization of certain forms one half of the sexual cells of each child 

 contains the determinants of the one parent, the other half contains 

 the determinants of the other parent. In thus showing that the 

 results of hybridization 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 experiments 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 well known, with Lamarck, and it is the great merit of Darwin 

 to have revived this idea. It is, however, remarkable that none of 

 the Darwinian authors seemed to consider it necessary that the 

 transformation of species should be the object 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 considered superfluous by those writing on the 

 hypothesis of evolution. Their scientific conscience was quieted by 

 the assumption that processes like that of evolution could not be 

 directly observed, as they occurred too slowly, and that for this 

 reason indirect observations must suffice. I believe that this lack of 

 direct observation explains the polemical character of this litera- 

 ture, for wherever we can base our conclusions upon direct obser- 

 vations polemics become superfluous. It was, therefore, a decided 

 progress when de Vries was able to show that the hereditary changes 

 of forms, so-called "mutations," can be directly observed, at least in 

 certain groups of organisms, and secondly, that these changes take 

 place in harmony with the idea that for definite hereditary char- 

 acteristics definite determinants, possibly in the form of chemical 

 compounds, must be present in the sexual cells. It seems to me that 

 the work of Mendel and de Vries and their successors marks the 

 beginning of a real theory of heredity and evolution. If it is at all 

 possible to produce new species artificially, I think that the dis- 

 coveries of Mendel and de Vries must be the starting point. 



