442 THE MAINTENANCE OF SPECIES 



works. Just as in the evolution of species during long periods of 

 geological time, variation must somewhere have entered in to make 

 it possible that an elephant and a mouse could have arisen from a 

 common, remote ancestor, so in the relay race of heredity we cannot 

 picture the details of how a succession of generations comes about, 

 when all the individuals concerned are alike. The uniform bulk of in- 

 heritance passes unnoticed. It is only the " sore thumbs " of variation 

 that stand out, for although hereditary succession may and does occur 

 in the absence of variation it is only when a visible variable is intro- 

 duced from one parent or the other, that we can see how the inherit- 

 ance of a characteristic jumps from one side of the house to the other, 

 skips a generation, doubles up, or behaves in some other manner. 

 One outstanding way in which hybrid variation is brought about in 

 nature is by sexual reproduction, in which two different streams of 

 germplasm unite to form a new generation. 



Pure hereditary strains, on the other hand, are probably not nearly 

 as common in nature as are hybrids. In self-fertilized plants, for 

 example, we may not expect to find much in the way of hereditary 

 variation, since no different outside germplasmal potentialities have 

 been introduced in the production of offspring. Likewise, in par- 

 thenogenetic organisms, which develop progeny without any contribu- 

 tion from the male parent, as well as in all kinds of asexual propaga- 

 tion, where a fragment of the parental body gives rise without germinal 

 modification to a new individual, one may expect to encounter 

 monotony so far as hereditary variations are concerned. Transient 

 variations that are induced by environmental causes, like the tanned 

 skin of a lifeguard at the seashore, or the luxuriant growth of a 

 pigweed on a manure pile, do not carry over in heredity. That 

 hereditary variations frequently do appear in the absence of hybrid 

 combinations is to be accounted for by the occurrence of mutations, 

 or spontaneous hereditary variations, which are mentioned in the 

 section on Evolution. 



In the early days of the nineteenth century, certain scientifically- 

 minded botanists in Europe began to explore the possibilities of 

 hybridization by artificially crossing plants. Koelreuter (1733-1806) 

 and Gaertner (1772-1836) in Germany, Naudin (1815-1889) in 

 France, and Knight (1759-1839) in England were conspicuous 

 pioneers in this field of experimentation. It remained, however, for 

 Gregor Johann Mendel (1822-1884) of Austria to become the master 

 hybridizer of them all, and to carry his experiments through to results 



