7 04 SCIENCE PROGRESS 



Half a century ago Mendel faced these difficulties and won 

 from a simple series of experiments results which we are now 

 beginning to realise are not merely invaluable to the breeder, 

 but which affect every subject in which heredity plays a part. 

 Mendel saw the weak point in all previous experiments, and in 

 the introduction to his paper on Experiments in Plant Hybri- 

 disation he stated that " not one [experiment] has been carried 

 out to such an extent and in such a way as to make it possible 

 to determine the number of different forms under which the 

 offspring of hybrids appear, or to arrange these forms with 

 certainty according to their separate generations, or to defin- 

 itely ascertain their statistic relations." In this sentence Mendel 

 outlined the whole plan of the series of experiments which have 

 placed the subject of inheritance on a definite basis. Thanks to 

 these, we now know that the phenomena which appeared almost 

 too complex for any analysis are in reality simple, and that, 

 given the knowledge of Mendel's laws, they may even be 

 predicted. Seven years ago the results of crossing together 

 two varieties differing in several characteristics would have been 

 summed up in the statement that " the type was broken," now 

 in many cases one can state so exactly what the progeny of the 

 hybrid will be that it is in the power of the breeder to make his 

 crosses with the certain knowledge of obtaining the results he 

 requires. 



From the first Mendel saw the necessity of keeping the 

 problems as simple as possible, and in his earlier experiments 

 he was satisfied to follow out the inheritance of single pairs of 

 differentiating characteristics, such as the round or wrinkled 

 shape of peas or the inflated or normal form of their pods. 

 Time after time he found these characteristics appearing among 

 the progeny of the hybrids in definite proportions. Thus he 

 found on the average that three round peas were produced 

 to each wrinkled pea or three individuals with normal pods 

 to one with inflated pods, and so on. Searching for a reason 

 for these statistic relationships, he was led to adopt the hypo- 

 thesis, which, later, he proved beyond all manner of doubt, that 

 the germ cells of the hybrids, i.e. the pollen grains and egg cells, 

 carried such characters in a pure and not a blended condition, 

 each gamete bearing one of the pair only. As will be shown 

 later, this simple fact, for such we may now term it, accounts 

 for the varied phenomena of hybridisation in an extraordinarily 



