Breeding experiments which show that hybridisation and mutation etc. 260 



is based the Mendelian conception of unit-characters. But it is not 

 possible really to isolate a unit-character from an organism as we may 

 isolate an element from a compound, so that in this respect the com- 

 parison between unit-characters and chemical compounds breaks down. 



But from another point of view there is a resemblance between 

 chemical compounds and hybrid characters which has been almost 

 entirely neglected in the recent literature. I refer to the fact that when 

 elements unite to form compounds or when compounds exchange their 

 atomic groups to form new bodies, the latter possess new and often 

 unexpected physical and chemical properties, which can only be pre- 

 dicted from a knowledge of the results of analogous reactions, and 

 not a priori from the properties of the combining elements themselves. 

 The probability that organisms when crossed will frequently show 

 corresponding phenomena, Mendelians have entirely ignored or over- 

 looked. But the intimate study of any unit-character enforces the 

 fact that it comes into expression as the resultant of the whole complex 

 of processes which constitute the organism's ontogeny. Even in such 

 definite cases as the one cited in this paper, where the presence of the 

 extra chromosome brings about the appearance of a certain type of 

 foliage, it is obvious that the extra chromosome by itself does not 

 produce the new type of leaf, but that this leaf-type together with 

 the other peculiarities of Oenothera lata developes as the resultant of an 

 original germinal complex containing 15 instead of 14 chromosomes. 



Wlao attempts to separate the hardness, or specific gravity, or 

 colour, or manner of crystallization of a given compound from each 

 other, or to consider them as separate and independent entities residing 

 in the compound? The attributes of organisms are similarly inseparable 

 from the individuals in which they find expression. The fact that certain 

 characters, for example, colour in petals, can be removed and substi- 

 tuted for each other in breeding experiments, does not justify the state- 

 ment that they can be "isolated", for in reality they have no existence 

 apart from an organism which, like the chemical compound itself, 

 contains many other "characters". 



The most striking resemblance between chemical processes and 

 alternative inheritance lies in the fact that in many cases, the direction 

 in which a reaction will "go" is determined by the proportions of the 

 reacting substances present, a slight change in these proportions some- 

 times producing a quite different end-result. I believe that in the same 

 way Mendelian characters, many of which obviously have a chemical 

 basis, are the end-products of metabolic processes, in which the origmal 



