D. J. SCOURFIELD OX MENDELIBM AND MICROSCOPY. 401 



128 possible constant combinations of the seven pairs of characters 

 with which he had started. 



The explanation which Mendel gave of the foregoing phenomena 

 was exceedingly simple, but nevertheless most ingenious. Taking 

 into consideration the everyday experience that in order to 

 maintain a constant race it is necessary that individuals hav; 

 similar characteristics should be mated together, he concluded 

 that the constant forms which he obtained from hybrids must ' 

 have been due to the mating of germ-cells containing similar 

 factors and no others. He assumed, therefore, that in the 

 formation of the germ-cells of hybrids, whether male or female, 

 the factors determining the different characters are segregated in 

 such a way that half the germ-cells contain only the factor for 

 one of each pair of characters, and the other half only the factor 

 for the other character — and this is Mendel's law. It has also 

 been called the Law of " Gametic Purity," a term which 

 emphasises the idea of the complete separation in the germ- 

 cells, or gametes, of the contrasted characters combined in the 

 formation of a hybrid. 



Mendel himself sawithat if his assumption were correct it ought 

 to be possible to predict what would be the result of crosses made 

 in various ways. For example, if a hybrid, instead of being self- 

 fertilised, were fertilised by one of the recessive parental forms, 

 then the progeny should no longer be produced in the ratio of three 

 dominants to one recessive, but in the ratio of 1 : 1 — i.e. both 

 characters in equal numbers. He showed by experiment that 

 this was actually the case. 



In order to get a general idea of the possibility of making 

 predictions by the help of Mendel's law, it is useful to set out 

 in detail the possible combinations which can take place with one 

 or more pairs of characters. Let us consider a crots between 

 round and wrinkled peas. The resulting hybrid plants contain 

 the factors for both characters, being, of course, the result of an 

 association of the two ; but, according to Mendel's law, when their 

 germ-cells are formed, the factors for the two characters are 

 separated again from one another, so that there are as many 

 pollen grains with one factor as there are with the other factor, ^ _ 

 and similarly with regard to the ovules. There are evidently jflKf 

 only four possible combinations in such a case— namely, $ R with ^ JT 

 ? R and with ? W 3 and S W with ? R and with ? W, where 



