THE ADAPTATION OF ORGANISMS 95 



the excess of female offspring, while relative starvation 

 tends to the formation of males. Indeed, the great 

 physiologist, Claude Bernard, went so far as to say 

 that the whole problem of evolution circled round 

 the variations in the nutritive factors affecting 

 plants and animals. 



Apart from the changes in the rate of response 

 of contractile organs immediately observable on 

 alterations in temperature already referred to, it 

 has been shown that the rate of multiplication of Physical 

 certain of the lowest animals is markedly increased ln 

 by a rise in temperature, whilst cold, in addition to 

 retarding movement, diminishes the rapidity of deve- 

 lopment and tends to induce the formation of dwarf 

 and even larval forms, and to affect the sex of flowers. 

 Similarly, light influences the formation of pigment 

 in certain animals, e.g., insects, and affects the 

 colouration of birds' eggs, while in relation to plants, 

 we have already quoted numerous instances of the 

 importance of variations in light in relation to the 

 distribution of chlorophyll, the anatomy and mor- 

 phology of leaves, the movements of motile leaves 

 and of free organisms. Light is also known to 

 govern the mode of reproduction in certain Algae, 

 and, in excess, to act injuriously on Bacteria, while 

 some botanists hold that deficiency in illumination 

 favours the production of male as opposed to female 

 cones in certain members of the pine family. 



The reaction of organisms of different types 

 on each other may be demonstrated by endless 

 examples. To quote only a few cases, we have the vital 

 alteration in form in both constituents due to the influ enees. 

 constant association of Algae and Fungi in the 

 composite structures we term lichens, the remarkable 

 cases of hypertrophy of vegetable tissue in fungal 



