EVOLUTION OF INTERSPECIES INTEGRATION AND ECOSYSTEM 



701 



were females, many of which were abnor- 

 mal. 



These relationships produce local fluctua- 

 tion of numbers in both parasite and host 

 populations and move toward eventual 

 equilibrium. Concentrations of egg laying 

 often give rise to local concentrations of 

 numbers. Galls of a given species of aphid, 

 cynipid, or other gall insect, are often 

 common in a small area or even on an in- 

 dividual plant, and are rare under similar 

 ecological conditions a short distance away. 



Some authors beheve that evolution 

 tends toward reduction of reproductive 

 potential in some forms such as those birds 

 and fishes that evolve protected nests for 

 the care of the young, or that nest in areas 

 with natural protection. Beebe (1906) said 

 many years ago: "The number of eggs 

 which a bird lays has been found to bear a 

 definite relation to the amount of danger 

 to which the species is exposed." Moreau 

 (1944) thinks that clutch size and mor- 

 taUty rate react upon each other and are 

 in mutual adjustment. 



Clutch size is characteristic of species of 

 birds (Averill, 1933; Stresemann, 1934, p. 

 373; Moreau, 1944). When the eggs in the 

 nest reach the number characteristic of 

 the species, the bird usually ceases laying 

 If, however, the eggs are removed by an 

 experimenter, many birds will continue to 

 lay until the number common to the species 

 is reached. In chickens, selective breeding 

 has increased the genetic capacity for egg 

 production. In species under natural condi- 

 tions, there would seem to be an intrinsic 

 psycho-physiological mechanism that main- 

 tains a number of eggs characteristic for 

 each species, this number being presumably 

 optimal for the species under the given 

 conditions. 



There are evolutionary tendencies to- 

 ward increased reproductive capacity and 

 greater density of populations in some 

 forms such as the termite, in which the 

 more primitive and less sociahzed species 

 have queens with low egg-laying capacity 

 that produce only a few (less than ten) 

 eggs daily, while in the more highly social 

 species, with much larger and better coor- 

 dinated colony populations, the queens 

 may lay several thousand eggs daily. The 

 phylogenetic relationships of these forms 

 leave no doubt that there was an evolu- 

 tionary increase in egg-laying capacity. 



The selection pressure toward augmented 

 reproductive potential in this case is prob- 

 ably mainly associated with the greater 

 social homeostasis (p. 672) possible with 

 large colony populations (see p. 274). The 

 conspicuous continued evolution of the 

 soldier caste toward increased defensive 

 adaptation against predators, together with 

 an increase in the proportional number of 

 soldiers in certain highly successful genera 

 (for example in Nasutitermes, Fig. 149), 

 is indicative of a fairly strong evolutionary 

 response to predation pressures. 



Egg-laying capacity has also certainly 

 increased during the evolution of parasitic 

 cestode worms from the free-Uving ances- 

 tral flat worms. In these parasites this evo- 

 lutionary tendency is probably connected 

 with the necessity for overcoming high 

 egg mortahty because of the sUght chance 

 of infesting the secondary host (Fig. 250; 

 p. 709). It is also interesting to note that 

 parasitic cuckoos lay many more eggs (thir- 

 teen to eighteen) than their nonparasitic 

 relatives (two to six). 



Invasion of the host body has probably 

 occurred through evolutionary stages that 

 gradually became more and more adjusted 

 to parasitism (Freeman, 1937; von Brand, 

 1946, pp. 279-284; see also p. 255). TaHa- 

 ferro (1948) suggests bacterial stages 

 leading to parasitism as follows: (1) the 

 free-hving putrefactive bacteria hving on 

 decaying matter, (2) the putrefactive or- 

 ganism hving in the lower intestine of 

 animals, (3) the tetanus organism Hving 

 on necrotic tissue, (4) true parasites such 

 as the typhoid bacillus, which is estab- 

 hshed in the body. Other evolutionary in- 

 vasions of hosts may occur through preda- 

 tion, seen in the phylogeny of mites, hce, 

 and fleas. Some parasites evolve from ex- 

 ternal to internal parasitism, for example, 

 the lung mites (Halarachne) of seals, the 

 lung mites {Pneumomjssus) of Old World 

 monkeys, and the chigoe fleas {Tunga 

 penetrans), the female of which burrows 

 under the skin of various animals, includ- 

 ing man. 



The fact that some parasites are in 

 reaUty adjusted to an interspecies predator- 

 prey relationship, rather than to a single 

 host species, indicates a long estabhshment 

 of such interspecies systems. The complex 

 hfe cycle (Cyclomorphosis, p. 685) of 

 malaria in man and anophehne mosquitoes 



