It is, however, more to our present purpose to discuss the _causes of variation_ than to lay down definitions of what variation is. One of the most obvious causes of variation lies in a change or changes in the environment. If we may assume, for the moment, that the type form of a species is the form which is the mean of all the extremes, and that this form is the resultant of all the varied forces brought to bear upon it, whether of food, climate, temperature, competition of numbers, soil, light, amount of water, etc., it will follow that any change in one or more of these forces, if continuous and considerable, any change, in other words, of the environment, will produce its effect upon the organism in question. And this effect will be for the better or for the worse, according to the particular nature of the change itself as tending towards, or away from, the _optimum_ of environment for the species concerned. Hence may be produced varieties, more or less marked according to the gravity of the change, although it must be noted that at times a change apparently unimportant from our point of view, will produce very marked results upon the species. It is indeed scarcely possible to predict with any certainty, in the present state of our knowledge (beyond certain broad results) what will be the particular effect upon a species of any given change in its surroundings. =Effects of Change in the Environment as tending to produce Variation.= (_a_) _Changes in Climate, Temperature, Elevation, etc._--In the eastern basin of the Baltic the marine Mollusca are much more stunted than in the western.[185] For instance, _Mytilus edulis_ near Kiel is 8–9 cm. long, while near Gothland it only attains a length of 3–4 cm. Mollusca living at only a shallow depth (_e.g._ _Tellina balthica_, _Mya arenaria_, _Cardium edule_) do not differ much in size in different parts of the Baltic, but in the far eastern basin the calcareous layers of the shells of _Mya arenaria_ and _Tellina balthica_ are extraordinarily thin, and disappear very rapidly after death, leaving only the cuticular membrane, still united by the ligament, in a perfect state of preservation. These remarkable variations are no doubt to a large extent due to the violent changes of temperature which are experienced in the Baltic, and by which the steady development of the animals in question is interrupted and thrown out of gear. The same species occur on the coasts of Greenland and Iceland, where they attain a considerably larger size than in the Baltic, in spite of the lower mean temperature, probably because their development is not interrupted by any sudden change from cold to heat or _vice versâ_.