80 The Mechanism of Evolution in Leptinotaksa 



activity in process and products. I see no escape from the conclusion that there 

 are not fixed and discontinuous units in the gametes, but rather that the results 

 that we observe in the soma, and with which we are able to deal in accurate 

 analytical and synthetic operations, are after all nothing more than reaction 

 systems of greater or less magnitude, capable of endless fragmentation, depend- 

 ing upon the nature of the materials and the conditions of the medium at the 

 time of combination. Each of these portions of the system as we observe its 

 somatic manifestations is symbolized for purposes of description, but is no 

 warrant for the concept of the existence of final indivisible units of any sort, 

 nor for discontinuity between them. In the material composition of the gametes 

 there can be no other discontinuity or isolation of the factors of composition 

 than that which exists in any physical substance or in a complex mixture of 

 substances. 



At present there is no evidence as to the nature of any gametic agent, and it 

 is quite possible that factorial differences may be due not only to actual differ- 

 ences in component materials, but also to differences in molecular arrangement 

 or to changes in reaction directions or in velocity. In fact, there are so many 

 possible modes of the production of factorial differences in materials that are as 

 complex as the living substance of the gametes that it is hardly worth considera- 

 tion at the present. 



The location of the different agents in the gamete is, and is likely to remain, a 

 question of debate for a long time. Cytological considerations and argument 

 have for many years placed all of the " bearers of the heredity characters " in 

 the chromatin, or at least in the nucleus, but there seems to be increasing evi- 

 dence, especially in the field of experimental embryology, that the cytoplasmic 

 portion of the gamete, especially of the egg, is also the bearer of hereditary 

 characters. These relate especially to form and symmetry, or to kindred deep- 

 seated phyletic qualities of the species. 



Depending upon the nature of one's perceptual experiences with these prob- 

 lems of organic phenomena and the initial philosophical viewpoint hinges the 

 concept of the structure of the germinal material that we create. 



METATHESIS OF GAMETIC AGENTS. 



Certain facts with regard to the behavior of characteristics of the parent 

 lines in crossing are becoming increasingly evident and certain. (1) Somatic 

 differences result from the combination of gametes of unlike potentiality in the 

 same character manifestation, and that out of many such combinations there is 

 interchange of the characters as far as the somatic manifestation is concerned, 

 and, therefore, we conclude interchange in the gametic masses also, and evi- 

 dence, from the derivation of pure stable lines in Fg showing the interchanged 

 characteristics, is from many observations in plants and animals, regarded as 

 proof that some gametic change has taken place. This is best described and 

 interpreted as interchange of gametic agents. (2) In all observed instances of 

 metathesis the interchange is always between equivalent characters of the same 

 kind, and occupying in the organism the identical location and manifestation ; 

 that is, flower characters of form or color or pattern interchange, leaf form or 

 plant habit interchange in plants, while in animals it is coat color, pattern, 

 activities, and so on of the same position and time of expression in the life of 



