140 THE FIRST PRINCIPLES OF HEREDITY 



gradual, continuous steps has thus been somewhat shaken, 

 while, on the other hand, De Vries's contention that larger 

 and sudden variations, called by him " Mutations," form 

 the material for Natural Selection, finds support in 

 Mendelism. 



The Mendelian law of inheritance has been verified in a 

 host of plants and animals ; but it must be well under- 

 stood that it applies in each case only to certain character- 

 istics of the organism, which form allelomorphic pairs, of 

 which one unit is dominant, the other recessive. With 

 regard to cultivated plants and domesticated animals, a 

 new era has dawned. Man is now able to unite given 

 desirable qualities in new favourable combinations. More 

 than that, undesirable characters can be bred out once for 

 all by rejecting those individuals which have the undesir- 

 able combination, without fear of losing the desirable 

 qualities, for the breeder can, in a Mendelian case, ascertain 

 with absolute certainty the underlying constitution of each 

 given individual. 



As for man, little progress has so far been made with the 

 application of Mendel's law. Some pathological character- 

 istics have been found to show Mendelian phenomena. In 

 addition, eye colour in man " mendelizes " (a short con- 

 venient teiTn for denoting breeding according to Mendel's 

 law). But we are only on the threshold of our knowledge. 

 More, much more, is to come yet, and the beginning made 

 augurs well for the future. 



Before finally leaving this subject we must slightly 

 touch on the relationship between the phenomena of 

 Mendelian crosses and those of Hybridism as given by 

 Weismann. Both Weismann and Mendel assume units of 

 characters (Weismann's Determinants), which in their 

 totality make up the given organism. But while in 

 Mendelian cases, to use Professor Thomson's words, " the 

 unit-characters are stable, and wiU not blend with other 

 contrasted analogous units, in other cases the unit-char- 

 acters are not so ' exclusive,' but ma}^ combine with 



