L52 MORGAN— THE CONSTITUTION OF [April 23, 



will be in the second generation. There is no other way as yet 

 discovered by means of which this relation can be predicted. 



If we compare our conception of the structure of the germ plasm 

 with that of Weismann we find in all of his writings except the last 

 one, that he supposed the chromosomes to be alike and that each 

 consisted of a series of ids that contained the totality of the de- 

 terminers that influence development. 



It is true that in his last writing he partially abandons his earlier 

 idea of whole ids for a conception nearer to ours of partial ids, — at 

 least for some of the determiners. In this respect his view more 

 nearly approaches the one here maintained. But even then his view 

 not being based on numerical data would leave us entirely helpless 

 in explaining the phenomena of inheritance in any particular case. 

 Without wishing in the least to detract from the value of Weis- 

 mann's brilliant speculation, nevertheless the difference in the way 

 in which the conclusions were reached in the two cases is one of 

 fundamental significance in all scientific work. Our view is based 

 on accurate numerical data that enables us to predict what any given 

 result in this field will be. It is this power to predict that gives 

 significance to a scientific theory. In this regard we believe that 

 our interpretation is a long step in advance of the purely imaginative 

 conception of the germ plasm that Weismann advanced. 



If now we bring our conception of the germ plasm to bear on 

 the problem of development we have a very dififerent view point 

 of that process from the one Weismann pictured. 



We think of every cell in the body containing one set of chromo- 

 somes received from the mother plus one set from the father. The 

 materials carried by these chromosomes influence development in 

 their entirety. Although we are able to localize certain materials 

 in the chromosomes that when present cause the eyes to be white, 

 and others that cause the eyes to be red, we do not mean that these 

 materials in the chromosomes go directly only to the parts that show 

 their influence more markedly. We mean that given one kind of 

 material and the rest of the cell there is elaborated a white eye; 

 given a different material in the same locus it produces, in con- 

 junction with the rest of the cell, a red eye. 



