^RACTICAL ASPECTS IN HEREDITY. 7 



In these brief remarks I have indicated some of the lines along which the 

 Mendelian discoveries will have a close bearing on the work of the practical 

 breeder. We have for the first time a conception of the true nature of at least 

 a part of the facts which underlie the outward and visible phenomena witnessed 

 by the breeder. As I have attempted to show, we have at last a clear notion 

 of the meaning of purity or fixity of type, of the consequences of dominance 

 and of the nature of heterozygous forms — phenomena which go to make up the 

 daily experience of those who are practically engaged in these pursuits. It is 

 impossible on the present occasion to go into many other fascinating problems 

 suggested by these simple facts. For example, we do not proceed far with 

 the practice of experimental breeding before we meet the phenomenon of the 

 decomposition or resolution of compound characters into simpler constituent 

 characters (hypallelomorphs), themselves possessing a measure of individual- 

 ity. Then again we are presented with a whole series of possibilities of the 

 utmost consequence both to the naturalist and the practical breeder. 



It is difficult to see this phenomenon of the decomposition or resolution of 

 compound characters without feeling the conviction that we have here the key 

 to a great part of the mystery of parallel variations. We are led to suspect 

 that the series of colors, for instance, into which the original color of the Car- 

 nation has been split up may be a similar series to that into which, say, the 

 sweetpea has been split up. We can in this way imagine that each series of 

 component colors consists of a number of definite terms related to each other 

 in a definite way such that, if we could ascertain the relation of yellow in 

 the one series, we could predict somewhat similar relations for yellow in the 

 other series. The colors of flowers give us many such series, and even classes 

 of series, of which some have obviously distinct laws of their own. Neverthe- 

 less, it is in a high degree likely that if one such series of colors were studied 

 statistically in such a way that what I have called the mutual relations of its 

 terms could be stated, we should have a model which would enable us to recon- 

 struct other similar series, to predict its terms, and possibly to set about pro- 

 ducing them at will. 



In this paper I have spoken only of the simpler deductions from Mendel's 

 principles. To this audience I need scarcely say that we are well aware that 

 those principles in their simple form cover only a part of the phenomena of 

 heredity. In trying to extend them or to cast them into a general form many 

 reservations must be made that cannot now be detailed, and a vast field must 

 be covered by specific experiment before such generalizations can be successful. 

 Chief, perhaps, of the difficulties we can at present foresee is that caused by the 

 existence of numbers of specific heterozygotes, which may appear quite unex- 

 pectedly owing to the presence of unknown differentiations between parent- 

 strains presumed to be identical. Such a case is that of the E. Henderson 

 mentioned above. Phenomena of this kind will doubtless be found elsewhere, 

 and will lead to great difficulties of interpretation. Against such cases the 

 observer must be on his guard. The significance of such forms can only be 

 studied by an analysis of their offspring. 



In addition to the general development of the inquiry we may note three 

 chief subjects that call for immediate investigation: 



