The Hydrozoa offer a complete series of gradations between those cases in which th term B is represented by a free, self-nourishing organism (e.g., Cyan&a], through those in which it is free but unable to feed itself (Caly cophoridce), to those in which the sexual elements are developed in bodies which resemble free zooids, but are never detached, and are mere generative organs of the body on which they are developed (Cordylophora).
In the last case, the " individual " is the total product of the development ot the impregnated embryo, all the parts of which remain in material continuity with one another The multiplication of mouths and stomachs in a Cordylophora no more makes it an aggregation of different individuals than the multiplication of segments and legs in a centipede converts that Arthropod into a compound animal. The Cordylophora is a differentiation of a whole into many parts, and the use of any terminology which implies that it results from the coalescence of many parts into a whole is to be deprecated.
In CordylopMra the generative organs are incapable of maintaining a separate existence; but in nearly allied Hijdrozoa the unquestionable homologues of these organs become free zooids, in many cases capable of feeding and growing, and developing the sexual elements only after they have undergone considerable changes of form. Mor phologically, the swarm of Medusa; thus set free from a Hydrozoon are as much organs of the latter, as the multi tudinous pinnules of a Comatula, with their genital glands, are organs of the Echinoderm. Morphologically, there fore, the equivalent of the individual Comatula is the Hydrozoic stock + all the Medusae which proceed from it.
No doubt it sounds paradoxical to speak of a million of Aphides, for example, as parts of one morphological indivi dual ; but beyond the momentary shock of the paradox no harm is done. On the other hand, if the asexual Aphides are held to be individuals, it follows, as a logical conse quence, not only that all the polypes on a Cordylophora tree are " feeding individuals," and all the genital sacs "generative individuals," while the stem must be a " stump individual," but that the eyes and legs of a lobster are "ocular" and "locomotive individuals." And this conception is not only somewhat more paradoxical than the other, but suggests a conception of the origin of the com plexity of animal structure which is wholly inconsistent with fact.
IV. Ætiology.
Morphology, Distribution, and Physiology investigate and determine the facts of Biology. ^Etiology has for its object the ascertainment of the causes of these facts, and the explanation of biological phenomena, by showing that they constitute particular cases of general physical laws. It is hardly needful to say that aetiology, as thus conceived, is in its infancy, and that the seething controversies, to whioh the attempt to found this branch of science made in the Origin of Species has given rise, cannot be dealt with in the limits of this article. At most, the general nature of the problems to be evolved, and the course of inquiry needful for their solution, may be indicated.
In any investigation into the causes of the phenomena of life, the first question which arises is, whether we have any knowledge, and if so, what knowledge, of the origin of living matter?
In the case of all conspicuous and easily-studied organisms, it has been obvious, since the study of nature began, that living beings arise by generation from living beings of a like kind ; but before the latter part of the 17th century, learned and unlearned alike shared the conviction that this rule was not of universal application, and that multitudes of the smaller and more obscure organisms were produced by the fermentation of not-living, and especially of putrefying dead matter, by what Avas then termed generatio cequivoca or spontanea, and is now called aliogtnesis. Redi showed that the general belief was erroneous in a multitude of instances ; Spallanzani added largely to tho list ; while the investigations of the scientific helmintholo- gists of the present century have eliminated a further category of cases in which it was possible to doubt the applicability of the rule " omnc vivum e vivo " to the more complex organisms which constitute the present fauna and flora of the earth. Even the most extravagant supporters of abiogeuesis at the present day do not pretend that organisms of higher rank than the lowest Fungi and Protozoa are produced otherwise than by generation from pre-existing organisms. But it is pretended that Bacteria, Torulce, certain Fungi, and " Monads " are developed under conditions which render it impossible that these organisms should have proceeded directly from living matter.
The experimental evidence adduced in favour of this proposition is always of one kind, and the reasoning on which the conclusion that a biogenesis occurs is based may be stated in the following form:—
All living matter is killed by being heated to n degrees.
The contents of the closed vessel A have been heated to n degrees.
Therefore, all living matter which may have existed therein has been killed.
But living Bacteria, &c., have appeared in these contents subsequently to their being heated.
Therefore, they have been formed abiogenetically.
No objection can be taken to the logical form of this reasoning, but it is obvious that its applicability to any particular case depends entirely upon the validity, in that case, of the first and second propositions.
Suppose a fluid to be full of Bacteria in active motion, what evidence have we that they are killed when that fluid is heated to n degrees? There is but one kind of conclusive evidence, namely, that from that time forth no living Bacteria make their appearance in the liquid, supposing it to be properly protected from the intrusion of fresh Bacteria. The only other evidence, that, for example, which may be furnished by the cessation of the motion of the Bacteria, and such slight changes as our microscopes permit us to observe in their optical characters, is simply presumptive evidence of death, and no more conclusive than the stillness and paleness of a man in a swoon are proof that he is dead. And the caution is the more neces sary iu the case of Bacteria, since many of them naturally pass a considerable part of their existence in a condition in which they show no marks of life whatever save growth and multiplication.
If indeed it could be proved that, in cases which are not open to doubt, living matter is always and invariably killed at precisely the same temperature, there might be some ground for the assumption, that, in those which are obscure, death must take place under the same circum stances. But what are the facts ? It has been pointed out at the commencement of this article, that the range of high temperatures between the lowest, at which some living things are certainly killed, and the highest, at which others certainly live, is rather more than 100 Fahr., that is to say, between 104 Fahr. and 208 Fahr. It makes no sort of difference to the argument how living beings have come to be able to bear such a temperature as the last mentioned; the fact that they do so is sufficient to prove that, under certain conditions, such a temperature is not sufficient to destroy life.
Thus it appears that there is no ground for the assump tion that all living matter is killed at some given tempera ture between 104 and 208 Fahr,
