520 REVIEWS — THE GENETIC CYCLE IN ORGANIC NATURE. 



and from each of these an embryonic fillet is produced, the farther development 

 of which gives rise to double embryos of various kinds.* 



It will be observed that the explanation here given of dublication 

 involves a principle like that called chorisis in plants, which is 

 employed, with more or less plausibility in different instances, to 

 account for increase in the number of floral organs. "We have seen 

 reason to reject its application in several instances where it has been 

 alleged by eminent botanists ; but we have never gone so far as to 

 deny that there may be instances in which it supplies the best 

 explanation of remarkable phenomena, and the probability would be 

 increased by a good analogy with what occurs in the animal kingdom. 



What follows occurring in connection with the notice of detached 

 sexual structures may suggest a new idea to some,, and the appended 

 note details a curious fact only lately established : — 



In fact, the whole question of detachment hinges on the proportionate 

 development of the somatic life, ?, e., the life of the body as one whole, and 

 the more or less independent life of its several organs, or what we may term 

 the topical or regional life. In the higher animals the special actions of the 

 several organs are as completely subordinated to that of the body as a whole,, 

 as are the powers of local corporations to the central government in any 

 well-ordered state, yet there still remains sufficient evidence of the real existence 

 of a distinct topical life. The hairs and teeth of animals generally, and the 

 antlers of the deer, have already been cited as furnishing illustrations of it. 

 The first set of teeth, for instance, are formed each in its own capsule by a 

 process of local growth, quite independent of that of the neighbouring tissues, 

 nay, in so far opposed to it, that at a certain stage of development the 

 integuments of the gum are partially disintegrated to allow of their eruption. 

 A tooth, thus generated by independent growth, some time after attaining 

 maturity, undergoes a process of decay, ending in its ultimate removal, when 

 a new tooth of the second dentition takes its place by a similar process of local 

 growth. In its turn this tooth also is shed, and though in most species it has 

 no successor, yet in a few there is a constant succession during the whole 

 lifetime of the animal ; and this is the general rule in the case of the hair^t 

 ■Hence in such local formations as teeth, hair, &c., we have, in the way they 

 are marked off from the neighbouring parts, and in this succession of growth, 

 maturation, and decay — repeated again and again, and epitomizing, as it were, 

 the life of the animal on which they grow — evidence of a vitality, quite as 

 defined perhaps in itself as that presented by the free zooids of the lower 

 species, though their functional dependence on the common circulation, and tho 

 mechanical bond of a common integument, prevent their exhibiting the more' 

 obvious phenomena of a separate life. But as we descend in the scale of 



• Aunals of Nat. History, -Ziid Ser., XVI., 49. 



T Paget's Lectures on Surgical Patholofjj'. Kirkes' Handbook of Physiology, Ch. X.. 



