66 CROSSING. 



can be expected to produce a 9 : 7 ratio, which exhibit the 

 character for the first time, and those of their off-spring 

 which happen to be heterozygous for both genes. We will see 

 in a later chapter how small the chances are, that a hetero- 

 zygosis for two genes will persist for a number of generations. 



If we see that polydactylous chickens, or black-skinned ones, 

 or animals with the feathers recurved, have only one gene 

 more than the normals with which we cross them, we may 

 assume with perfect safety, that in all these cases, such genes, 

 as are new to the species, are derived from other species, which 

 may not have shown the peculiarity. The main point to remem- 

 ber is, that new characters, new dominant characters, can 

 originate by crossing, in all those instances in which the simul- 

 taneous action of two genes on the development results in 

 some thing different from that of either alone. 



The circumstantial evidence upon which Bateson would feel 

 obliged to concede the possibility of a spontaneous origin of 

 a new gene, loses its weight as soon as we decide to look upon 

 genes as upon things which may, but need not, influence the 

 development. It is quite inadmissable to speak of charicters 

 which are determined by two genes, in contrast to characters 

 that are determined by one gene. Every single character of any 

 individual must, in so far as it depends upon the genotype, the 

 heredity, be determined by quite a long list of genes, acting 

 on all the different stages of development, and so influencing 

 this, that at some stage the character, the quality studied 

 results. We know the genes only through the difference their 

 presence can make in the development of an organism, as 

 contrasted to the development of another organism, which 

 lacks them. In some instances we know one gene in this way, 

 in other cases we may know to or three genes, which, when 

 cooperating to the development of a line of organisms affect 

 the final result on the same point. When Baur and one of us 

 came out in favour of the so-called presence and absence theory, 

 Plate criticized us. He had the impression that we believed that 

 a definite quality could result from the absence of a gene, and 



