212 ON VARIABILITY AND ADAPTATION 



sentative and capable of conveying all the paternal or maternal 

 properties. 



Let us pass back a generation. The germ cells of the parent 

 are similarly formed by a combination of chromosomes from the 

 two grandparents. These must be represented, as indeed must 

 also be the chromosomes of the great-grandparents, etc. What 

 is more, in the process of reduction he holds that certain chromo- 

 somes derived from certain ancestors are cast out, others retained, 

 and that it is the variation in the series of retained chromosomes 

 that in the main determines the variations between the members 

 of the same generation. The hereditary substance in the fertilized 

 ovum thus consists of several complexes of primary constituents 

 contained in the chromosomes, or " idants," each of which 

 complexes (an "id") comprises within itself all the primary 

 constituents of a complete individual. 



Each "id" Weismann conceives as being composed of a 

 mass of different kinds of parts, each of which bears a relation 

 to a particular part of the perfect animal, and so to some extent 

 represents its primary constituents, although there may be no 

 resemblance between these Anlagen and the finished parts. 

 These representatives of individual parts contained in each 

 " id " he terms " determinants " (Bestimmungsstilcke). In 

 each " id," therefore, there must be as many determinants as 

 there are regions in the fully formed organism capable of in- 

 dependent and transmissible variation at all stages of develop- 

 ment for the caterpillar stage, for example, as well as for the 

 butterfly ; determinants even for the egg, because eggs, cater- 

 pillars, and butterflies are seen to vary independently. If, as 

 has been noted, the individual hairs on the antennae of insects 

 are capable of transmissible variation, and if, in man, the con- 

 formation of particular teeth is inheritable, then determinants 

 there must be for these individual hairs and teeth. If a little 

 patch of scales on the butterfly's wing is peculiar to one variety, 

 for those few scales there must be a determinant. 



And lastly, each individual determinant must be made up 

 of molecules of living matter, or biophores. Such biophores, he 

 holds, must be larger than any chemical molecule ; they must 

 consist of groups of molecules, some large and complete, others 

 simpler and more minute ; so that ultimately it is the interaction 

 of these biophores the casting out of some from one parental 



