262 PARASITISM [ANIMAL. scarcity of food or of extreme pressure from enemies, vari ous methods of solving the problem of life would be attempted, the successful results of which in a few cases persist especially in ectoparasitism, not the least obvious mode of retaliation on stronger foes. The degree of the parasitism is, as we have seen, not of primary moment, and its intimacy may be increased. There are naturally some physiological limits of respiration, &c., determining the possibilities of parasitism air-breathing insects are found on land animals or at most on some amphibian forms, water-breathing Arthropods on water-breathers, water- breathing worms only in the interior of land animals ; but even these limits may be overstepped by adaptation when, for example, the respiration becomes cutaneous in Penta- stomum, Sarcoptes, &c. The various modes of transmission of parasites, though of great practical importance, do not call for much dis cussion here. They may be summarized as follows after Leuckart : (1) the majority of parasites reach their hosts through the medium of food or drink ; (2) eggs are in some cases transferred from one animal to another by actual bodily contact, e.g., the eggs of Pentastomum by the licking of dogs ; (3) sometimes the eggs are deposited in or on the host by the mother, for example, by insect para sites, such as Ichneumons, (Estridse, cfec. ; (4) in some rare cases parasites are transmitted by self-infection,- for example, young Trichinx, born free in the alimentary canal of their host, bore their way thence directly into the muscles, there to grow into the well-known encapsuled worms. Eggs or proglottides of tape-worm may, on gain ing the exterior, be transmitted inadvertently to the mouth, and so recommence their life-cycle within the same host. The mode of diffusion of the ova of parasites presents many analogies to that of seeds in the vegetable kingdom : thus wind and water are alike utilized, passing animals may serve as unconscious bearers, and the like. Though well protected by a usually thickened egg-shell and an often remarkable degree of vitality, so as to resist pro longed drought, burial, and other vicissitudes, the parasite has an exceedingly small chance of success in finding a host ; to preserve the species from extinction an enormous number of eggs must be produced, far exceeding that of free-living organisms. Thus Leuckart points out that as a tapeworm has an average lifetime of two years, and pro duces in that time about 1500 proglottides, each contain ing say 57,000 ova, and since the species is not increasing in numbers, an ovum has thus only one chance in 85,000,000 of reaching maturity. The difficulties are of course increasingly greater as the life-history becomes more complicated, demanding an increasing number of hosts. Given a sufficient number of eggs, however, no difficulty is insuperable, and few parasitic forms accordingly seem in any risk of disappearance, except, it is to be hoped, in the case of civilized man and the domestic animals, where the large consumption of cooked food, aided by conscious hygienic precautions and medical aid, tends to exclude or remove them. Effects of Parasitic Life on Parasites. So far from treat ing the phenomena of parasitic life as highly aberrant, and the peculiarities of parasitic form as differentiations sui generis, it becomes evident that we have to do with only one of the many cases in which the influence of environment on organism is clearly marked. The aetiology of parasitism is only a fraction of a vaster general ques tion; and we shall never fully understand the adaptation of the parasite to its host until the relation of environment to organism has been far more profoundly analysed and completely experimented on inquiries which have only recently begun to be seriously set on foot. The most cur sory consideration of the action of environment shows how profoundly it determines form ; of this ho better examples can be found than those furnished by the habit of plants. It is easy to see how submerged leaves must become dissected, or desert plants tend to become succu lent ; how evergreens are only possible in certain condi tions of climate, or thorns are only useful where herbivor ous mammals abound. In the same way we can broadly see that the conditions of life profoundly influence animal form. Before considering how the abnormal parasitic environment affects the parasite, we should know how the normal environment affects the non-parasite, and how the two cases differ. The environment thus needs analysis into its factors, the organism similarly into its constituent systems of organs ; and the influence of any factor of the environment upon each system and organ demands deter mination, species by species, before safe and exhaustive generalizations can be obtained. Pending these inquiries, which are destined to take so large a place in the biology I of the future, and within the present narrow limits, only the merest outline can be attempted. Morphological science has but slowly and with difficulty disentangled itself from the primitive classifications of plants and animals by habit and resemblances of external form the physiologist, however, needs to reassert the claims of these and develop them in detail ; as for the child so for him whales are in a sense fishes, and bats birds, just as the swimming organs of the former, like those of the penguin or cuttlefish, are all fins, or the flying- organs of the latter and those of insects are wings alike. Such considerations show too the first importance of the mechanical conditions, primarily those of locomotion or rest, and whether in water, or land, or in air, since these determine, not only external form, but muscular and skeletal disposition and structure. These determined, conditions of heat and light play an obvious part ; copious supplies of heat energy to the organism have a distinct result in stimulating plant growth, and accelerating that of animals ; light too, a primal necessity for green plants, has also the most marked effect on animals, which develop tracts of absorbent pigments in its presence, these becoming locally evolved for perception into eyes ; while in relation to sound-vibrations and impressions of contact other sense organs develop. Quantity of food has its influence mainly on size, but nature of food and mode of feeding demand many appropriate specializations of details of form. Expressing the same adaptations from the other point of view, that of the organism, we see how not only the general form but the integument with its colour and texture, and also the respiratory and alimentary organs, are necessarily fitted to avail themselves of the given con ditions ; how the circulatory and how the reproductive systems must comply ; how the sensory organs must take note more and more of the changes in the environ ment ; and how the whole series of complex adaptations demands a similarly complex internal mechanism for their co-ordination through the nervous system. From the slightest analysis then of the relation of organism to environment, the theory of evolution might almost have been predicted, since, if the details of environ ment and organism be indeed obviously and precisely adapted one to another, change in the former must either be followed by the extinction of the latter or its modifica tion in the requisite details. To explain the modus operandi of change in the organism, we have mainly to boar in mind Dohrn s admirably expressed " principle of functional change," the simple conception that any living tissue, however specialized, still retains traces of all the functions of living protoplasm, and that any one of these traces may be indefinitely increased by favourable conditions, and the
