SHARE OF THE PARENTS IX BUILDING UP THE OFFSPRING 43 



more than three generations back impossible, and as such reversions 

 undoubtedly occur, we must conclude that manifold new combinations 

 of the paternal and maternal chromosomes take place. This obviously 

 happens during the maturation-divisions, at least in the Metazoa. 



The more numerous the rods or the free individual ids in 

 a species are, the more numerous are the possible combinations. 

 Whether all the mathematically possible combinations actually' occur 

 is a different question, which I should not like to answer in the 

 affirmative just yet ; but in any case the actual number of com- 

 binations in a species with many nuclear elements will be greater than 

 in one with few, and in this respect those species in which the ids 

 occur as independent granules will have an advantage over those in 

 which they are combined into rods or bands (idants). These latter, 

 however, afford us a better possibility of deducing the new com- 

 binations of the ids, although the idants themselves are not outwardly 

 distinguishable from each other. 



I must refrain from going into these highly interesting processes 

 in more detail just now. So much is certain, that Nature makes use 

 of various means to bring about the re-combination, and at the same 

 time the reduction of the ids during the two ' reducing divisions.' 

 This is proved by the fact recently established l)y Montgomery, that 

 in many animal groups reduction results from the first maturing 

 division. Whether it operates at this stage with rings, bands, double 

 rods, X-shaped structures, groups of four (tetrads), and so on, all this 

 serves the same end, the more or less thoroughgoing re-arrangement of 

 the hereditary vital units. I am convinced that new investigations 

 into these processes, if they were undertaken from this point of view, 

 would lead to very important results ^. It would be important to find 

 out how great the variations are which thus arise, for it is very 

 probable that they differ in degree in the different animal-gTOups. 

 Even the comldnation of the ids into rods (idants) indicates that some 

 species may be more conservative than others in maintaining their 

 id-combinations, and that there will be among them a greater tenacity 

 in the hereditary combinations of characters (i.e. of the ' type ' of the 



* Since this was written for the first edition ohsel•^•ations of this process have been 

 considerably increased, and discussions as to the exact interpretation of these are in 

 full tide ; we are surrounded l>y a wealth of new observations, facts, and explanations, 

 without having attained to a consistent and unified theory. Several. natui-alists, such 

 as Boveri, Hacker, Wilson, and others, have attempted interpretations, but these are 

 in many points contradictory to one another. It is thei'efore impossible to enter into 

 the question in detail here ; further light from new observations must be awaited. So 

 much we may say, however, that it is not chance alone which presides over the 

 re-arrangement of the chromosomes during the reducing divisions ; affinities play a part 

 also ; there are stronger or weaker attractions between the chromosomes, which aid in 

 determining their relative position to one another'. 



