182 HEREDITARY CHARACTERS 



It appears that we have two opposite modes of trans- 

 mission. One in which certain characters are definitely 

 segregated, and for which it is necessary to assume entities 

 which are distributed in an alternative manner to the 

 gametes when new individuals are produced. Another in 

 which characters apparently blend when crossed. 



Having thus apparently two very different ways in 

 which characters may be transmitted from parent to off- 

 spring, it becomes necessary to inquire whether any dif- 

 ference can be distinguished between the characters that 

 are transmitted in one way, and those that are transmitted 

 in the other. 



The most striking point about the Mendelian characters 

 is that almost all that have been subjects of experiment, 

 have been in domesticated races of animals or plants. On 

 the other hand, when natural races are crossed, we find 

 differences in characters are generally blended. Between 

 these two extremes, the Mendelian and the blending char- 

 acters, we have many gradations. It is very suggestive 

 that the cases which show the most perfect segregation 

 are those in self-fertilising plants, while departures from 

 the Mendelian law become more and more frequent when 

 we come to organisms where the gametes must necessarily 

 be derived from two separate individuals. It is necessary 

 only to refer to the difference in the results obtained 

 from crossing characters in peas and poultry to realise 

 this. 1 Many of the gradations between segregation and 

 blending have already been dealt with in the explanation 

 that was offered with regard to the cases of incomplete 

 segregation. It is obvious that the segregation of entities 

 must be most perfect in self-fertilisation. Even in self- 

 fertilising plants which have been domesticated for a long 

 period of time, some of the characters do not behave as 

 did the characters in Mendel's original experiments with 

 peas. For instance, two peas were crossed in one of which 

 the starch grains were "potato-shaped" and large and single, 

 1 See pp. 173, 176. 



