6io ON INCREASE IN COMPLEXITY [pt. in 



at 6, 25 and 90 days after fertilisation of the egg, and a complex 

 which modifies one of these effects may have no effect on the other 

 two, e.g. if colour is replaced by albinism, yellow is entirely sup- 

 pressed but senile adiposity is not influenced. 



Plunkett made a systematic analysis of the way in which genetic 

 factors and environment interacted to produce bristles during the 

 embryonic development of Drosophila. These bristles can easily be 

 counted, and so afford a quantitative variable. What actually 

 happens finally as regards bristles depends on (i) genetic factors, 

 e.g. missing-bristle genes, extra-bristle genes, etc., (2) the tempera- 

 ture during the developmental period, (3) other environmental 

 conditions such as nutrition, (4) differences of internal environment 

 and (5) random internal variations. The last two of these can be 

 avoided by suitable methods. Rise of temperature, Plunkett found, 

 tends to suppress bristle formation. It must act, then, either by 

 decreasing some bristle-forming reaction or by increasing some 

 bristle-inhibiting reaction. If the temperature characteristic, he 

 argued, of this reaction is less than development as a whole, the 

 former must hold, but if it is greater, then the latter must hold. As 

 the evidence came out, it was in favour of the former theory, for the 

 critical thermal increment was 36,400, a value often found for heat 

 destruction of enzymes, although for development as a whole it was 

 27,800 from 14 to 17°, 17,100 from 17 to 25° and 9000 from 25 to 30°. 

 By various calculations, Plunkett showed that the reaction in question 

 began very early in the larval stage, and took place in three steps: 

 [a) production of R (a destructive agent for the enzyme) from some 

 protoplasmic component, {b) destruction of B (enzyme from the 

 gene) by R and [c] the formation of the bristles catalysed by B. 

 A bristle-reducing gene might therefore throw its weight into the 

 catalysis of the formation of R. Plunkett suggested that all genes act 

 by differential acceleration of enzyme actions during embryonic de- 

 velopment. Similar work on temperature characteristics of gene 

 action has been done by Nadler; and Gowen, after 'K-ra.ying Drosophila 

 at various stages of development, reported that the abnormalities 

 produced, being strictly confined to certain groups of cells, showed 

 that the gene had been affected prior to any action, and while it 

 was still, as it were, lying latent in the nucleus. 



Huxley & Ford and Ford & Huxley have undertaken interesting 

 studies on the eye-pigment of Gammarus chevreuxi. Allen & Sexton 



