96 



A PRIMER OF BIOLOGY 



and insect galls, and the structural changes induced 

 in some sponges by the constant living with them 

 of certain polyps. The varied forms of flowers are 

 now very generally looked upon as direct adaptations 

 to visits of insects and that the manifold forms of 

 domestic plants and animals have arisen as a result 

 of conscious selection and cultivation by man is a 

 fact too familiar to require proof. Evidence in 

 abundance is forthcoming in Darwin's classic work 

 on the subject ("Animals and Plants under Domesti- 

 cation") and in the extensive literature that has 

 arisen since its publication. 



The conditions of the environment are infinitely 

 varied in different parts of the world even, it 

 may be, in the same district. In no two regions in- 

 deed are they exactly similar in all respects, and even 

 in the same spot, the conditions are never the same 

 for two moments in succession. Under these circum- 

 stances, it must be obvious that the organism, 

 whether plant or animal, must be capable of keeping 

 itself in equilibrium or accord with the ever-changing 

 conditions. In certain regions some conditions of 

 the environment are specially emphasised. Thus, 

 in a desert region the absence of water is the principal 

 factor to be considered, and unless the plant is adapted 

 to live in such dry conditions it must obviously 

 succumb. Again, aquatic plants are adapted to 

 live either entirely or partially submerged. A 

 general survey of the plant world enables us to dis- 

 tinguish certain types of structure specially adapted 

 to special climatic conditions. Thus we have aquatic 

 plants, desert plants, arctic and alpine plants, 

 seacoast plants, swamp plants, &c., as well as plants 

 adapted to peculiar modes of life, such as parasites, 

 carnivorous plants, climbers, epiphytes, these latter 



