EVOLUTION, CYTOLOGY AND MENDEL'S LAWS. 225 



not learn from them the nature of that process, but one of its limits. 

 Moreover, the probability that these laws are of general application is 

 greatly lessened by the fact that they are demonstrable only in con- 

 nection with narrowly inbred and much divergent varieties of plants 

 and animals, to which condition of the experiment the phenomena 

 discovered may prove to be due, rather than to any general fact or 

 mechanism of heredity. It seems certain, however, that neither theory 

 nor experiment will make permanent progress in this direction as long 

 as we continue to confuse under the word hybrid several extremely 

 diverse evolutionary conditions, and fail to realize that generalizations 

 based on any one kind or type of hybrids are quite premature and 

 irrational. 



Sterile Hybrids. — The original notion of a hybrid, or at least the 

 most popular meaning of the term, is that of a cross between organic 

 types so widely diverse that the progeny are in some way abnormal or 

 defective, especially with reference to reproduction. Among animals 

 sterile hybrids can not be propagated, but in plants they can be grown 

 from cuttings or buds, and are thus preserved as horticultural 

 * varieties. ' 



Aberrant Hybrids. — The second and succeeding generations of 

 hybrids not completely sterile often show striking deviations from 

 both parental types. As these new characters are analogous to the 

 abrupt variations of close-bred plants described by Darwin as 'sports' 

 and more recently renamed 'mutations' by De Vries, it has been sug- 

 gested that they ma}^ be due to the same causes, that is, they may not 

 be in reality the result of crossing, but rather of an inadequate con- 

 jugation or defective fertilization which allows a lapse from the normal 

 form. Both mutations and mutative hybrids are comparatively in- 

 fertile, so that their suddenly attained new characters should not be 

 looked upon as true contributions to evolutionary progress. 



Reciprocal or Mendelian Hybrids. — Mendel and his successors have 

 proved that there is still a third type of less abnormal hybrids, in 

 which there is no permanent combination or averaging of divergent 

 parental characters, although it is not known that vigor and fertility 

 are notably diminished. Mendel's so-called laws are generalized state- 

 ments of the results of his experiments upon the crossing of different 

 garden varieties of the pea; he himself found that the same was not 

 true among hybrids of Hieracium.* A part of the scientific com- 



* The question has been raised as to whether Mendel's discoveries should 

 be called ' laws.' The present view would deny to them universal application 

 as ' laws ' or ' principles ' of heredity, though it admits as probable their gen- 

 eral truth for a certain evolutionary condition or stage. 



Laws of gases are not called laws of matter, and do not apply until 

 matter reaches the gaseous state. Similarly, there can be no objection to 



VOL. LXIII. — 15. 



