PART III 



ANIMAL ECOLOGY 



CHAPTER XXIX 



THE STRUGGLE FOR EXISTENCE, ADAPTA- 

 TION, AND SPECIES-FORMING 



TECHNICAL NOTE. Multiplication, or increase by geometric 

 ratio, among animals can be illustrated by noting the many eggs 

 laid by a single female moth or beetle or fly or mosquito or any 

 other common insect (or almost any other non-mammalian animal). 

 The production of many live young by each female rose aphid can 

 be readily seen ; the number of young in a litter of kittens or pups 

 or rabbits is a good illustration. From this geometric increase it is 

 obvious that there must be a great crowding of animals and a strug- 

 gle among them for existence. This struggle and the downfall of 

 the many and success of the victorious few can be observed by 

 rearing in a small jar of water all the young of a single brood of 

 water-tigers (larva of Dyticus} or other aquatic predaceous insect. 

 The strongest young will live by killing and eating the weaker of 

 their own kind. In a spider's egg-sac the young after hatching do 

 not immediately leave the sac, but remain in it for several days. 

 During this time they live on each other, the strongest feeding on 

 the weaker. Thus out of many spiderlings hatched in each sac com- 

 paratively few issue. This can be readily observed. Open several 

 egg-sacs and count the eggs in them. Let the spiderlings hatch 

 and issue from some other egg-sacs belonging to the same species 

 of spider. The number of issuing spiderlings will always be much 

 less than that of the eggs. The actual working of natural selection 

 and the forming of new species can of course be seen only in re- 

 sults, and not in process. The great variety of adaptation, the fit- 

 ness of adaptive structures, can be readily illustrated among the 

 commonest animals. Animals showing certain striking and unusual 

 adaptations will perhaps make the matter more obvious. To all 

 teachers will occur numerous opportunities of illustrating, by refer- 



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