ORIGIN AND SIGNIFICANCE OF SPINES 91 



the plant body as affording a more primary conception of the 

 causes and modes of spine growth among climbing plants. 



In many cases of retrogressive series of animals there 

 seems to be a close parallelism with some of the characters 

 observed among the climbing plants. If the Ammonite fam- 

 ily during the Cretaceous, or near the close of the Mesozoic, 

 is taken as an example, it cannot be said that the environ- 

 ment of these old-age or pathologic series is unfavorable in 

 respect to food, temperature, etc., for with them are asso- 

 ciated many vigorous progressive series of other organisms. 

 Neither can it be said that in many cases the animals perished 

 on account of over-specialization, though this was evidentlj' 

 the cause of the extinction of a large number. The return 

 to a condition of second childhood in old age cannot be 

 called a progressive specialization, since it clearly jDoints 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 Cephalopoda show a simi- 

 lar attenuation of the shell, evincing a stoppage of growth 

 in the animal. Among the Mollusca it seems quite likely 

 that attenuation of form often accompanies decreased growth 

 power. 



The pathologic varieties of the Steinheim Planorbis, as 

 described by Hyatt, ^^ or of the recent Planorbis eomplanatus 

 described by Pir^,^" are further illustrations of this attenua- 

 tion 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 



