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On comparative longevity in man and the lower animals
On comparative longevity in man and the lower animals Author:Edwin Ray Lankester Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: A. Longevity In Organisms Generally. 3. Longevity defined. It is very necessary to have a clear perception of the meaning of the principal terms involved in ... more »the consideration of the duration of life. By ' longevity' must be understood the length of time during which life is exhibited in an individual.1 Unless we introduce the term ' individual,' and assign to it a definite meaning, we become involved in numerous difficulties when making a comparison of the length of life in different species of organic beings. However important in a zoological sense the definition of ' individual' may be which regards the various forms and existences appertaining to a species between ovum and ovum, as the individual of that species, for physiological purposes, such a definition cannot be accepted. The whole product of a fertilized germ, whilst it no doubt, in many cases, agrees with all requirements as a definition of the individual of a species, is yet, in many other cases, open to .muchobjection. In the Vertebrata, in Mollusca, in most Insects, such a definition is unobjectionable; but when we consider the numerous examples of asexual reproduction we are led into difficulty. ' It seems a questionable use of language to say that the countless masses of Anacharis alsinastrum which, within these few years, have grown up in our rivers, canals, and ponds are all parts of one individual.'i And yet, as this plant does not seed in England, these countless masses, having arisen by asexual multiplication, must be so regarded, if the above definition be accepted. In the Hydrozoa are we to ascribe the same amount of individuality to those forms which give rise to separate polypidoms by the separation of budded offspring, as to those to which these remain attached, to form a compound polypidom ? The same...« less