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Show 476 PROF. G. GULLIVER ON [June 15, The present Tables, though so extensive, show how numerous are the Vertebrates of which we still require an examination of the blood-corpuscles. The sum of the facts at this time known on the subject is so far imperfect, that we are ignorant of how soon new ones may turn up to subvert even our most cherished theories or generalizations. Hence the remarks or deductions on the present occasion, being confined to the measurements given in the Tables, must be considered as provisional, subject to modifications or corrections to suit the advance of knowledge, especially as regards Fishes, Batrachians, Cetaceans, Sirenians, and some other Vertebrates. It is desirable, too, that m y measurements should be subjected to experimental examinations by independent observers. To rigorous accuracy these Tables, of the averages of a far greater number of measurements, have no pretension. In this respect all that can be candidly said is, that, though they have been carefully deduced from innumerable and generally correct observations of the corpuscles, the size of these is by no means invariable in a single species, and that, even were they ever so constant in magnitude, seeing how much they usually differ among themselves in every field of vision, commonly to the extent of one third larger or smaller than the mean, their average dimensions could not be easily determined with sure precision. Upwards of a third of a century has passed since Dr. Bowerbank, experimenting with a cobweb micrometer at one of his delightful and instructive entertainments, found a remarkable difference in the size of the red blood-corpuscles obtained from the fingers of three gentlemen among his guests then present together. In the human subject I have often observed similar diversities, though to a less extent than appeared in Dr. Bowerbank's experiments; and I have notes of results to the like effect of observations, long since made by me, on single and on different individuals of one and the same species of all the Vertebrate classes. But such variations (in Man, see further p. 484), whether in a single individual or in different individuals of one species, are confined within such limits as not to prevent good approximations to the truth in the measurements. It should also be borne in mind that small organisms, even when each of them has a fixed diameter, vary so much among themselves that it would be difficult, if not impossible, to determine with absolute precision their mean dimensions, however easy it may be to measure truly a single blood-disk or a spore of a cryptogam. For example, let any person make trials with several portions of one and the same sample of spores or little seeds of a plant, when the results of numerous true measurements will fail to afford precisely the same average diameter. How, then, can this be expected of objects so variable in size and shape as the red blood-corpuscles 1 Those of Mammalia, when dried slowly, are apt to become misshapen and more or less irregularly contracted ; but when dried instantaneously in single or very thin layers on a glass slide, their form is admirably preserved, and their size is a shade larger than in the wet state, especially when prepared in summer. The pyreneematous corpuscles, |