1 86 DYNAMICS OF LIVING MATTER 



The observations of Mendel have since been confirmed and enlarged 

 upon. Not only botanists like De Vries, who independently rediscovered 

 Mendel's laws, but also zoologists like Bateson and his pupils, Cuenot, 

 Castle, Guyer, and many others, have added to this field. 



Mendel's laws do not, however, include all the cases of hybridization. 

 De Vries has investigated this field in a masterful way, and has shown 

 that there are at least two types of hybridizations : one in which, as in 

 Mendel's cases, a separation of the discriminating characteristics occurs 

 again in the offspring, and another in which constant races are at once 

 produced. This type of hybridization is the one which proves especially 

 useful to plant breeders in their attempts to produce new varieties. 

 De Vries believes that the latter type of hybrids is produced when the 

 sexual cells of one parent have a determinant for which there is no cor- 

 responding determinant in the sexual cells of the other parent. 



The objection might be raised that such a theory of chemical deter- 

 minants in the sex cells as the cause of heredity might find difficulty 

 in explaining the heredity of instincts; I believe that the contrary is 

 the case. In a paper on " Egg Structure and the Heredity of Instincts" * 

 I have pointed out that the hereditary character of the instincts demands 

 a chemical rather than a morphological theory of heredity. Many 

 instincts are obviously the outcome of tropisms. For the transmission 

 of an instinct based on heliotropism, all that is required is the presence 

 in the sexual cells of photosensitive material, or of a substance from 

 which such material can be formed. 



The current morphological and cytological literature contains many 

 attempts at explaining the phenomena of heredity on a purely morpho- 

 logical or cytological basis. There is no objection to this, as long 

 as we realize that the morphological structures can only play a role 

 through their physical and chemical properties. 



2. THE DETERMINATION OF SEX AND THE SECONDARY SEXUAL 



CHARACTERS 



Several years ago an embryologist published the hypothesis that sex 

 could be determined by submitting the mother to a certain diet. Delage 

 pointed out that this idea was contradicted by the fact that in about 

 30 per cent of the cases twins have different sex, which would be 

 impossible if the diet of the mother Determined the sex of the offspring. 

 There is, however, one condition under which twins have invariably 

 the same sex ; namely, when they come from the same egg. We have 

 seen in a former lecture that from one egg twins can arise; namely, 



* Loeb, The Monist, 1897. 



