THE BEARERS OF THE HERITAGE 305 



individual differences of such organisms. In this connection it is a 

 significant fact that in young hybrids between two distinct species the 

 early stages of development, especially as regards symmetry and 

 regional specifications, are exclusively or predominantly maternal in 

 character, but the male influence becomes more and more apparent as 

 development progresses until the final degree of intermediacy is 

 attained. 



From the evidence at hand this much seems sure, that the paternal 

 and maternal chromosomes respectively carry substances, be they 

 ferments, nutritive materials or what not, that are instrumental in 

 giving the final parity of personal characters which we observe to be 

 equally heritable from either line of ancestry. It is clear that most 

 of the characters of an adult organism cannot be merely the outcome 

 of any unitary substance of the germ. Each is the product of many 

 cooperating factors and for the final outcome any one cooperant is 

 probably just as important in its way as any other. The individual 

 characters which we juggle to and fro in our breeding experiments seem 

 apexed, as it were, on more fundamental features of organic chemical 

 constitution, polarity, regional differentiation, and physiological 

 balance, but since such individual characters parallel so closely the 

 visible segregations and associations which go on among the chromo- 

 somes of the germ-cells it would seem that they, at least, are repre- 

 sented in the chromosomes by distinctive cooperants which give the 

 final touch of specificity to those hereditary characters which can be 

 shifted about as units of inheritance. 



Sex and heredity. Whatever the origin of fertilization may have 

 been in the world of life, or whatever its earliest significance, the 

 important fact remains that to-day it is unquestionably of very great 

 significance in relation to the phenomena of heredity. For in all 

 higher animals, at least, offspring may possess some of the character- 

 istics originally present in either of two lines of ancestry, and this 

 commingling of such possessions is possible only through sexual repro- 

 duction. As has already been seen, in the pairing of chromosomes 

 previous to reduction, the corresponding members of a pair always 

 come together so that in the final segregation each gamete is sure to 

 have one of each kind although whether a given chromosome of the 

 haploid set is of maternal or paternal origin seems to be merely a 

 matter of chance. Thus, for instance, if we arbitrarily represent the 

 chromosomes of a given individual by ABC, abc, and regard A, B and 

 C as of paternal and a, b, and c as of maternal origin, then in synapsis 



