372 GENERAL HISTOET OF THE INFUSORIA. 



tions of plants, small aquatic animals, organic debris, and other substances in 

 the water may conceal in their cavities or interstices either mature animalcules 

 or their immature or encysted forms. Further, we know the air to be per- 

 meated by animalcular life ; that every wind wafts organized beings, for the 

 most part in an encysted state, together with germs and spores, animal and 

 vegetable, to and fro ; and every exposed collection of water, unless protected 

 by the most careful and complex contiivances, must perpetually receive fresh 

 colonists. Kow, among all these matiu'e, encysted, immature, and embryonic 

 inhabitants of a portion of water, existing in it when first submitted to obser- 

 vation, or subsequently introduced into it from without, there must neces- 

 sarily be a constant change in their relative abimdance, and even in their con- 

 tinued existence in it. Matm-e individuals may die out, be devoured by other 

 animals, or be otherwise destroyed before miiltipljing themselves, or may, 

 by encysting and reproduction, develope beings of a different general character, 

 {^ e. undergo a real transformation; encysted beings may merge into hfe, 

 immatui-e and embryonic forms take on their perfect conformation ; hidden 

 organisms may come out from their concealment ; or the new ones borne by 

 the air may manifest themselves ; and in these and other conceivable ways 

 new series of inhabitants may make their appearance on the scene. 



Lastly, the succession of species is greatly influenced by the changing con- 

 ditions of the water and its contents, by atmospheric conditions — cold, heat, 

 and electricity, and the moisture or dryness of the air. All the facts collected 

 under the head of Habitats indicate the mutual relations between the appear- 

 ance of certain animalcules and the jDresence of particular plants or even of 

 certain animals, or the existence or absence of decomposing organic matter. 

 We have, moreover, so to speak, carnivorous and herbivorous Ciliata, each 

 and all severally requiring their special nutritive elements in the water. 

 From these circumstances it is evident that particular species will disappear 

 when the conditions favourable to them fail, to be in all probability replaced 

 by others to which the change is favoiu^able and necessary ; for instance, the 

 vegetable feeders will decrease and disappear when the minute plants on 

 which they feed are consumed ; so those animals requiring pure water will die 

 out when decomposing organic matters multiply, and will be replaced by the 

 forms which delight in their presence, but have remained undeveloped until 

 the conditions favoui'able to their existence are brought about. The little 

 CoJeps (to give a particular illustration) delights in the eggs and contained 

 substance of Entomostraca, and makes its appearance in company with those 

 animals, without which it is only occasionally seen. And it remains to be 

 noted, that unless an animalcule is duly supphed with appropriate nourish- 

 ment, its reproductive powers remain in abeyance, and consequently its whole 

 race may vanish from this cause. 



A particular example of the succession of species may be quoted from 

 Cohn's essay on Eeproduction of Infusoria {Zeitschr. 1851, p. 2^'6). In a 

 vessel containing decomposing Sphogyra, at first appeared countless speci- 

 mens of Paramecmm Aurelia ; these were replaced by the Proteus of Baker, 

 either the Lacrymaria Proteus or the Tracheloce7xa Olor (Ehr.) ; these in 

 their turn were followed by ChUodon CucuUulus, and after a few days by a 

 Colpoda ; afterwards large Euplotes with prominent green globules, probably 

 a new species, and lastly, colourless specimens of Euplotes Charon exhibited 

 themselves, — all these species following each other in succession in the course 

 of three weeks, a new form appearing on the decline of a preceding, attaining 

 its maximum in number, and then decreasing in its tiu"n to make room for the 

 next in the series. Moreover, this excellent observer remarks that a similar 

 •succession is observed in the case of microscopic plants, such as OseiUatoria. 



