70 BIOLOGY 



animals. The lack of colour pigment on the under- 

 surface of flat-fish such as the plaice, flounder and dab, 

 we know from Cunningham's experiments to be directly 

 connected with the absence of the light rays. One 

 thing is certain, that it is a determining factor in deciding 

 the mode of reproduction in many of the Algae. Further, 

 the variations in light have considerable influence on the 

 anatomy and morphology of leaves, and on the move- 

 ments of many free organisms. 



The fourth set of environmental factors the direct 

 influence of organism on organism plant on plant, 

 animal on animal, or plant on animal, has been demon- 

 strated again and again in the extensive literature with 

 such a wealth of example that it is necessary to mention 

 here only a very few. We may quote, for example, the 

 deformation caused in the structure of sponges by other 

 animals living in or with them ; the injurious effects of 

 certain parasites on their hosts, and the slight modifica- 

 tions they induce in their host's structure ; the changes 

 in the habits and structure of the various Algae and 

 Fungi we find associated together in the structures we 

 call Lichens ; and the fact that the varied forms of 

 flowers are but adaptations to the types of insects that 

 visit them. 



The most important vital force which affects the 

 organisms, whether for good or ill, is man. His influ- 

 ence is easily recognised when we realise that the varied 

 forms of domesticated plants and animals are the pro- 

 ducts of his conscious selection. 



Let us glance briefly at the care of offspring, a topic 

 connected with the subject of this chapter. This is seen 



