must, in each period, have given rise to innumerable multitudes of what have been called 'trees of life,' branching out into animal and vegetal forms of almost inconceivable variety, Myriads of these trees,' including all their branches and innumerable ramifications, may have wholly died out during the many vicissitudes of the earth's surface and the long lapse of ever fruitful ages; though the descendants or ultimate ramifications of some of such trees dating back to quite different and perhaps far-distant epochs-may still survive. How far, however, the roots of any of the 'trees' from which the existing higher forms of life are derived, may have extended back into the depths of geologic time, we are utterly unable to estimate.

Throughout all this life-evolving period of the history of our globe, the progress of organization' seems to have been essentially similar. And that this should be so, seems readily explicable by the consideration that living things, both as regards their origin and their subsequent differentiation or development, are the immediate products of ever-acting natural laws or material properties. These properties should act therefore now as they have ever done, and so continue to produce almost similar effects.

The lower the forms of life-that is, the nearer they are to their source-the greater seems to have

been the similarity amongst those which have been produced in different ages-just as the lowest forms are now similar in all regions of the earth. On the other hand, the longer any particular 'tree of life' has lived (of which there have been countless multitudes born in each age), the wider may be the divergence of form presented by the ultimate outgrowths of any two of them, or of outgrowths of similar rank produced from trees which have developed during different ages-especially when the assemblages of organisms constituting one of these ideal trees, have lived under the influence of any unusual set of telluric conditions.

The vertebrate' grade of organization may have been many times attained by ultimate branches of different 'trees of life.' But how long or when the particular tree of life,' from one of the branches of which Man was developed, appeared upon the earth, it is quite impossible to say.

II.

ON THE TEMPERATURE AT WHICH BACTERIA, VIBRIONES, AND THEIR SUPPOSED GERMS ARE KILLED WHEN IMMERSED IN FLUIDS

OR EXPOSED TO HEAT IN A MOIST STATE.

FOR more reasons than one we may, perhaps, now look back with advantage upon the friendly controversy carried on rather more than a century ago between the learned and generous Abbé Spallanzani and our no less distinguished countryman Turberville Needham. Writing concerning his own relation to Needham, the Abbé said, "I wish to deserve his esteem whilst combating his opinion"; and in accordance with this sentiment, we find him treating his adversary's views with great respect, and at the same time repudiating much of the empty and idle criticism in which so many of Needham's contemporaries indulged with regard to his work. This criticism, Spallanzani says, "Without looking into details, contented itself by throwing doubt upon some of the facts, and by explaining after its own fashion others whose possibility it was willing to admit." He moreover warmly reprobated the ignorant and disrespectful

Nouvelles Recherches sur les Découvertes Microscopiques et la Génération des Corps Organisés, &c. London and Paris, 1769, vol. i., p. 69. + Loc. cit., p. 9.