266 CAUSES OF DEGENERATIVE EVOLUTION 



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is an angular region, the sides of which are 

 prominent, and bear leaves modified into spines 

 to serve as protecting organs, as in the similar 

 case of Cereus. Higher up the branch the pro- 

 minent sides become flat, and the 

 spines are replaced by minute 

 scales. This degeneration is the 

 result of loss of utility. The 

 Phyllocacti are epiphytes, and 

 their situation consequently places 

 them out of the reach of crop- 

 ping animals. The spines near 

 the basis of the branches are a 

 survival from the terrestrial an- 

 cestors of Phyllocactus. The 

 spines higher up have degenerated. 

 Animals offer many instances 

 of atrophy as a result of inutility, 

 both in individuals and in species. 

 (4) Atrophy of the 'branchial 

 arches in mammals. As they are 

 no longer functional, most of 

 the mammalian branchial arches 

 atrophy. Three pairs alone persist, 

 and of these it is only those parts which are useful. 

 (5) Atrophy of ventral fins. Instances of atrophy 

 through uselessness in species are to be found in 

 the ventral fins of fishes like the Pediculati, which 

 live in the mud, or in Protopterus, which for a part 

 of the year is completely buried in mud (fig. 1 9, p. 44). 



FIG. 78. Branch of 

 Phyllocactus crenatus. 



