CHROMOSOMES AND HEREDITY — DARLINGTON 419 



any soft Lamarckian adaptation. The Lamarckian principle, we must 

 remember, was at that time generally maintained by naturalists and 

 physiologists and assumed by medical and social scientists. Even phi- 

 losophers had their opinions, and they were on the Lamarckian side. 

 Yet there was no room in this picture for the Lamarckian emblems, 

 the giraffe, the salamander, or the midwife toad. 



The more specific objections to the chromosome theory were also very 

 various. The assumption that the chromosomes were alone responsible 

 for heredity left a gap in our theory of development. Is not heredity 

 merely a repetition of development? Yet this theory of heredity 

 almost ignored development, and it was based on a single organism — 

 a fly with a most disorderly development of its own. The chromosome 

 theory also left a gap where the cytoplasm should be — where indeed 

 European workers had found evidence of determination. Loeb, with 

 his unfortunate idea that the cytoplasm carried the solid basis of 

 heredity while the nucleus bore only a few frills, provided a line of 

 defense for weaker opponents of the chromosome theory. 



Again it was pointed out, quite rightly, that the gene mutations of 

 Drosophila could not be representative of natural variation for they 

 were in their effects both disadvantageous and discontinuous. In the 

 first respect they contradicted the helpful mutations of the evening 

 primrose, Oenothera. In the second respect they failed to explain 

 the universal property of continuous variation. As for the chromo- 

 somes themselves, did they not at the end of eveiy cell division dis- 

 solve and disappear into that bag of fluid, the nucleus? As for 

 crossing-over, the foundation of Morgan's interpretation, it was sup- 

 posed to happen only in one sex and not in the other. But who had 

 ever seen it happen anywhere? Crossing-over, like the genes them- 

 selves, was a stroke of fancy, a mathematical artifact invented to 

 salvage a broken hypothesis. 



With regard to these chromosomes, it was true, there were a variety 

 of accounts of what they did [4]. The two cell divisions known as 

 meiosis, when the germ cells were formed, were especially disputable. 

 Some believed that there were general niles; others that there were 

 many kinds of meiosis in different groups of plants and animals. 

 Most believed that, if there was a rule, it was that the corresponding 

 chromosomes from the two parents paired as threads side-by-side. 

 But a few stoutly maintained that chromosomes in the nucleus were in 

 an endless chain which split up crosswise into segments to give 2a; 

 single chromosomes at mitosis, x double chromosomes at meiosis, and, 

 by a freak of nature, 4a3 chromosomes at mitosis making a new tetra- 

 ploid [5]. Clearly this view was of no help to those who believed 

 that the chromosomes made heredity and were differentiated in linear 

 structure; but it was a help to those who did not think anything of 



