SEX AND HEREDITY 471 



and genera, and so on. By gradual steps one may thus pass from the 

 similar to the strongly dissimilar. Within certain limits the dis- 

 similar can breed together : for instance, individual strains and 

 varieties commonly interbreed. Sometimes species and even genera 

 can produce offspring together. The bi-generic crosses of certain 

 Orchids artificially produced are a well-known feature of horticultural 

 shows. In seed-bearing Plants the seeds produced by crosses from 

 distinct parentage may germinate, though where the parents are 

 strongly dissimilar either there is no offspring, or it is apt to be 

 itself sterile, as is the mule. Thus a limit is set upon distinctness 

 of parentage. Such crosses between parents more or less dissimilar 

 are called hybrids, and they are characterised by sharing in greater 

 or less degree the qualities of both parents. So far as qualities can 

 be transmitted from parent to offspring, the mechanism of sexual 

 fusion offers the opportunity for their transmission. There is no 

 doubt that Heredity is a fact ; but the sexual transmission of char- 

 acters is governed and limited by certain laws which restrict its 

 scope. The result is that while the sexually produced offspring 

 shares the characters of both parents, it practically in all cases differs 

 in some degree from each of them. It is a matter of common observa- 

 tion that it does not repeat all the characters of both parents. 



The characteristics of organisms have been classed under two heads : 

 those which are heritable and are transmitted, and those which are 

 not. The former includes such features as have not been referred 

 directly for their production to the impress of external circumstances 

 upon the parent. The latter include those features which can be so 

 referred. It may, however, be a question how far these categories 

 are mutually exclusive. As examples of those not transmitted 

 mutilations may be quoted ; or the immediate accommodations of 

 the growing parts to the impress of gravity, light, etc., such as 

 are described in Chapter VIII. However effective these may appear 

 to be in determining the mature form of the parent, there is no 

 sufficient evidence that these specific results are handed on to the 

 offspring. They are described as fluctuating variations. But there 

 are other larger or smaller deviations from type, appearing 

 suddenly and individually, which have not been referred directly 

 to known causes, and are found to be heritable. These have been 

 called "Mutations." The numerous minor deviations from type 

 in Draba verna, described by Jordan, are cases in point. The 

 fluctuating variations appear to leave no permanent impress upon 

 the organism, so as to affect its gametes. The heritable mutations, 



