Environment, a modern term for the influencing surroundings of an organism. Neither plant nor animal can be understood as a rounded-off unity; the whole life or function is made up of action and reaction between the organism and its environment. Streams of matter and energy from without preserve the relative constancy of the organism, as of a special wave-crest in the sea; while changes in the streams have their corresponding changes within. The plant or animal has obviously a strong unity of its own, but even that is in part due to ancestral welding under the hammers of the environment. It may seem, too, to vary of itself like a fountain in the air, but throughout all its rises and falls there blows the wind of the environment. The influence of outside conditions has been recognised by most naturalists from the time of Hippocrates, and is taken for granted in our everyday speech and action. There is considerable difference of opinion, however, as to the importance and degree of this influence. Thus Buffon, Treviranus, and Geoffroy St Hilaire regarded the surroundings as directly hammering changes on the organism; while to Erasmus Darwin and Lamarck internal changes arose as indirect functional results of new environment. Charles Darwin allowed a measure of truth in both these positions, but emphasised the indirect destructive action of the animate and inanimate environment in the struggle for existence. These three positions are still held, some emphasising one, others another, the majority combining the three (see EVOLUTION).
The external influences may be classified in four groups: (1) Molar or Mechanical; (2) Chemical; (3) Molecular or Physical; (4) Vital or Animate. A few instances of each kind of influence will be given; for details, the reader must be referred to Semper's Animal Life, and an appendix paper to that classic work.
(1) Molar or Mechanical Influences.—Confined space brings about a dwarfed brood. Currents mould the sponges and corals, chisel the shells, cut up the water leaves; and the wind blows the trees out of shape. Vertical pressure may broaden out the form or produce latent life; while the very weight of structures effects many changes, from the mode of an egg's dividing to the slope of a tree's branches. On the whole, however, the mechanical hammers effect least.
(2) Chemical Influences.—Subtler but more potent are the influences which we group together as chemical. Good aeration—i.e. abundant oxygen—quickens development and stimulates the whole life. The 'vital combustion' is more rapid. In drought some animals become desiccated (see DESICCATION), and the Axolotl (q.v.), removed from the watery bed into less moist environment, becomes an Amblystoma. The composition of the medium seems to have a great influence on the forms and phases of simple unicellular life, and even a comparatively high animal like the Brine-shrimp (q.v.) was converted by Schmankewitsch from one species into another by gradually altering the salinity of the water. Food may be included under this group, and its influences are the greatest. To Claude Bernard the whole problem of evolution was very much a question of variations in nutrition. It is the food, to a large extent, which conditions the cyclic changes in cell-life from an active ciliated phase to a passive amoeboid or encysted one (see CELL). Parasites in their passivity illustrate a nemesis at least exaggerated by their copious nutrition. Abundant food is associated with growth and asexual multiplication; the plant sends out its runners, the plant-lice continue their rapid parthenogenesis (see APHIDES), but a check to nutrition hastens the sexual climax. Good nutrition tends towards the production of female offspring; by increasing the quantity and quality of food Yung nearly doubled the normal percentage of females among his tadpoles; while starved caterpillars and tadpoles tend to turn out mostly males. According to Sutton, hypertrophy of one part over another has been the principal process in the transition from hermaphroditism to unisexuality, while Barfurth emphasises the importance of fasting as a progressive factor in evolution. From the cell-cycle to the colour of canaries' feathers, the importance of the food-factor is widely demonstrable.
(3) Physical Influences.—Heat stimulates growth and hastens the reproductive maturity, while cold tends to dwarf and often produces coma. Maupas increased the rate of reproduction in an Infusorian five times by an elevation of 17° C. in the temperature of the water. Dallinger has recently described how the monads which he succeeded in educating to stand a temperature high above the usual fatal point became not a little altered at the crisis of each successive elevation. Weismann has shown how an artificial winter induces the winter colouring of a certain Butterfly (q.v.). The sunlight which kills the floating germs has a beneficent action on most organisms, very notably on plants. In some cases it seems to affect colour, and Poulton has altered the colour of his Caterpillars (q.v.) by the tint of the glass under which they lived, or has produced golden chrysalids by keeping them in gilded boxes. Electric and probably magnetic energies seem to have influences, but these cannot yet be said to be intelligible.
(4) Vital Influences.—In the web of life organisms influence one another indirectly in a hundred ways, but they have likewise their direct influences. There are animate as well as inanimate 'hammers.' Sea-spiders specifically alter Hydroids, and Semper also notes the mutually influencing associations of a snail and a coral, of an Annelid and an Antipathes (a genus of Zoantharia). Crustacean parasites may very effectively alter their crustacean hosts, and internal guests have also marked results. Nor have insects worked about plants for millennia without leaving direct mechanical imprints of their footsteps. Finally, man's influence in Domestication (q.v.) may be mentioned.
Hundreds of cases of external influence, now on one system, now on another, are known, but few are thoroughly understood. The observation of the results is one thing, their rationale another. In a general way, barring purely mechanical modifications, the influences are referable to two classes—one set increasing constructive processes, storage of energy, passivity, anabolism; the other set increasing destructive processes, expenditure of energy, activity, katabolism.
The susceptibility of organisms to outside influences is very varied. Simple and young forms are evidently more in the grip of external circumstances than are complex and adult organisms. The cell, the seedling, the child, are more impressive than the worm, the tree, the man. Passive forms also, like sponges and algæ, corals and trees, are much more under the tyranny of surroundings than active organisms with devices at command for parry or escape. The vegetative system, again, is oftener affected than the reproductive, and thus doubtless many environmental influences affect only the individual 'body,' and are not transmitted to the species.
The degrees of influence are also very varied. The dints of the environment may be deep or superficial, very direct or very remote in their results. A primary influence from without may have more than one result within the organism, by starting correlated variations. The influence may remain without apparent result in the individual, and yet the nemesis may be evident in the offspring. Influences may also accumulate within the organism without any evident outcrop for a time, but at length an indirect, at first sight spontaneous, change may be the far-off result. The influence of environment in forcible distribution, in a cataclysmic destruction of local fauna, and the countless indirect results of changed conditions must also be kept in view.
The action of the environment as a factor in organic evolution is very variously estimated by different schools of naturalists (see EVOLUTION). It is still undecided how far characteristics impressed upon an individual organism by outside forces can be transmitted. If the influence send its roots deep into the organism, it may probably affect the reproductive elements, and thus become transmissible; but according to Weismann and others, no facts demonstrating such cases are as yet known. It must not be forgotten, however, that no organs of the body are really insulated, and that changes from without may be transmitted by infinitesimal degrees for each individual generation, till they eventually, if the conditions remain constant, accumulate into a specific character. In opposition to Weismann's view, Professor G. H. T. Eimer's work (1888) on the Origin of Species, as based on the inheritance of acquired characters according to the laws of organic growth, should be consulted (see HEREDITY).
Finally, as there is no doubt as to the importance of environmental conditions for at least the individual organism, the influence of human environment must be recognised as a biological fact of the greatest social moment.
See Semper's classic work, The Natural Conditions of Existence as they affect Animal Life (Inter. Sc. Series, 1880); G. H. T. Eimer, Die Entstehung der Arten (1888); J. Arthur Thomson, 'Synthetic Summary of the Influence of the Environment upon the Organism,' Proc. Roy. Phys. Soc. Edin. ix. (1888), pp. 446-99.