The Biology of Senescence 



species are concerned, and not 'envisaged' by evolutionary 

 teleology. It could be argued that once gonadal senescence has 

 become established in a species, eventual somatic senescence is 

 a rule inevitable from the withdrawal in post-reproductive 

 life of the selection-pressure towards homoeostasis. A clear 

 physiological link between the activity of the gonad and the 

 growth and survival of the animal has been demonstrated in a 

 few forms (e.g. Daphnia, Edlen, 1937, 1938), although even in 

 Daphnia, oogenesis continues until death (Schulze-Robbecke 

 1951). In many vertebrates, however, even total castration has 

 little or no adverse effect on longevity, and may increase it. 

 Although senescence of the gonad is, in an evolutionary sense, 

 the most important form of ageing, it is not self-evident that in 

 the artificially-protected animal it must always be followed by 

 generalized somatic senescence, unless the two processes are 

 causally related. Such an identification reflects, once more, a 

 human preoccupation. Reproductive function, because of its 

 ease of measurement, remains at most a justifiable test of con- 

 tinuing vitality in old animals — fertility at least indicating the 

 absence of irreversible organ changes in one important system. 

 Metabolic decline, either measured directly by calorimetry and 

 manometry, or inferred from reduction in spontaneous activity, 

 has also been regarded as an index of senescence — often on 

 theoretical grounds, as representing the accumulation of in- 

 active 'metaplasm' at the expense of active protoplasm (Kasso- 

 witz, 1899, etc.) or the completion of a 'monomolecular 

 autocatalytic reaction' such as that postulated by Robertson 

 (1923) or Bertalanffy (1941). The decline of heart rate in 

 Cladocerans (Ingle, Wood and Banta, 1937) has already been 

 mentioned. The mean resting heart rate in man also tends to 

 decline throughout foetal and postnatal life. Child measured the 

 age of hydromedusae by the decline in their rate of pulsation 

 (Child, 1918). In some invertebrates (planarians, Child, 1915; 

 hydromedusae, Child, 1918; molluscan adductor muscle, Hop- 

 kins, 1924, 1930) and in some isolated vertebrate tissues (arti- 

 cular cartilages, Rosenthal, Bowie and Wagoner, 1940, 1941, 

 1942; rat blood vessels, Lazovskaya, 1942, 1943; avian muscle, 

 Glezina, 1939; rabbit muscle, Cheymol and Pelou, 1944; rat 

 brain homogenate, Reiner, 1947; liver, kidney and heart homo- 



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