34:8 C. E. Bteclier — Origin and Significance of Spines. 



age or pathologic series is unfavorable in respect to food tem- 

 perature, etc., for with them arc associated many vigorous pro- 

 gressive series of other organisms. Neither can it be said, that 

 in many cases the animals perished on account of over-spec- 

 ialization, though this was evidently the cause of the extinc- 

 tion of a large number. The return to a condition of second 

 childhood in old age cannot be called a progressive specializa- 

 tion, since it clearly points to a deficiency of growth force. 



Old age types, or phylogerontic forms, among animals may 

 show the same attenuation or suppression of the body as do 

 climbing plants. Thus, Baculites, considered by Hyatt as a 

 typical phylogerontic type, has a very attenuate shell, and some 

 species, after attaining a certain diameter, cease to increase in 

 any direction except length. On account of being a chambered 

 shell, it is manifest that the growth of the animal must have 

 practically ceased, while its secretive activities were continued 

 and confined largely to lengthening the shell. Other related 

 genera of Cephalopods show a similar attenuation of the shell, 

 evincing a stoppage of growth in the animal. Among the Mol- 

 lusca, it seems quite likely that attenuation of form often 

 accompanies decreased growth power. 



The pathologic varieties of the Steinheim Planorbis, as 

 described by Hyatt, 35 or of the recent Planorhis complanatus 

 described by Pire, 57 are further illustrations of this attenuation 

 accompanying the uncoiling of the shell. The sedentary 

 Magilus, immersed in its coral host, is also an example, for 

 not only does the shell cease to increase in diameter, but the 

 whole interior, except a small cavity at the end, is filled with 

 a solid deposit of lime. Similar examples could be multiplied 

 indefinitely. Since, however, but few of them are spiniferous, 

 their consideration does not properly come within the scope of 

 the present discussion, though, as is well known, some of the 

 attenuate forms often enlarge and contract periodically, such 

 enlargements frequently leaving prominent laminae or nodes 

 that are sometimes differentiated into spines. They suggest 

 the observations on growth, senescence, and rejuvenation, made 

 by Minot, 48 who showed that in guinea pigs from a very early 

 age, the increments of growth are in a steadily decreasing 

 ratio to the increase of weight of the animal. This led to the 

 general conclusion, that the whole life of an individual is a 

 process of senescence or growing old. 



Spines arising by a real pathologic or diseased condition of 

 the individual can have little or no effect in producing a normal 

 spiniferous variety or species. However, some note should be 

 taken of them, especially as they may be congenital, and thus 

 appear through several generations. In the human species, the 



