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R. Ruggles Gates . 
weight of the roof; so alternate breakdown and repair have led to 
the present structural relationships in many parts of the human 
body. Thus the heart is described as an aneurism, and the stomach 
as a dilatation sac, whose thick muscular wall has developed as the 
result of disaster and repair. It is assumed that the transmission 
of these changes probably takes place by a morphogenetic reply 
in utero to increased functional stresses. 
Brief mention must be made of the important work of 
Harrison (1920a) on melanism and other racial characters in 
certain Geometrid moths. After intensive observation and 
experiment with the genus Oporabia, Harrison concludes that the 
subspecies O. filigrammaria was evolved from O. autumnata during 
the Glacial period by the action of changed climatic conditions, 
and that “ almost certainly” many of the racial and subspecific 
differences are “ true Lamarckian effects,” in particular the food 
instincts in O . filigrammaria and the period of emergence of the 
pinewood race of O. autumnata. Moreover, a Lamarckian 
explanation of melanism is considered necessary, the darkening 
resulting from metallic salts in the food, derived from the smoke 
in industrial areas and from sea fogs in coastal areas where 
melanism frequently occurs. Melanism is known to behave as a 
simple Mendelian dominant in many species (see e.g Onslow, 1920). 
Harrison (1920b) has recently shown this to be true of the melanic 
variety of Tephrosia crepuscularia when crossed with the type, out 
in interspecific crosses between T. crepuscularia and T. bistortata a 
chaotic series of F 2 and F 3 forms was obtained, the behaviour 
being no longer describable in Mendelian terms. This recalls the 
result obtained by the writer (1915f) in crosses between the 
Mendelian dominant character in (Enothera rubricalyx and another 
species, (E. grandiflora. In both cases dilution and modification of 
the character has resulted from crossing with a different species. 
If crossing will modify such a unit-factor, then it is not unreasonable 
to suppose that they may be capable of environmental modification. 
If the Lamarckian explanation of melanism is a true one, then 
it would appear to be a case of parallel induction, because of its 
manner of inheritance in crosses, indicating that the nuclear 
structure is already altered. The function of hormones as an 
evolutionary means of handing on or accelerating modifications, as 
suggested by Cunningham (1908), Dendy (1911) and others, indicates 
a method by which modifications may be perpetuated until the 
change becomes germinal by producing an alteration in the nucleus. 
