EVOLUTION OF SPINES. 45 



foot, in the Papuan species the number is generally reduced 

 to three. The wide structural differences separating the 

 egg-laying mammals from all other members of their class 

 render it almost unnecessary to observe that the spines 

 of the echidnas are an entirely independent development. 

 Like so many of the spined mammals, the echidnas have 

 extremely short tails and thick bodies, with the neck 

 indistinctly marked. We have thus decisive evidence 

 that a more or less complete coat of spiny armour has 

 been independently acquired in the following groups of 

 mammals, viz., in the porcupines, mice, and octodonts 

 among the rodents ; in the hedgehogs and tenrecs among 

 the insectivores ; and in the echidnas among the egg- 

 layers. From the perishable nature of these appendages 

 we have, unfortunately, no evidence as to the existence of 

 spines among fossil mammals ; but from the foregoing 

 considerations, we are strongly inclined to think they may 

 be mainly characteristic of later epochs. 



In this connection it is interesting to notice that the 

 spiny globe-fishes (Diodon, &c.), often termed " sea- 

 hedgehogs," in which the spines are bony and therefore 

 capable of preservation, do not date back below the 

 Tertiary, and that spiny fishes are unknown in earlier 

 epochs. Moreover, some although by no means all of 

 the palaeozoic sea-urchins appear to have had very minute 

 spines. Hence it would rather seem as though the 

 history of spines has been exactly the opposite of that of 

 bony armour, which has tended gradually to disappear with 

 the advance of time. 



Finally, we have to notice the general similarity in 

 appearance of so many of the more specialized spiny 

 mammals, due not only to their bristly coat, but likewise 

 to the general shortness or absence of the tail, and the 

 rounded, plump form of the whole body. The bearing of 

 this independent development of spines in so many groups 

 of mammals, together with the acquirement of a general 

 external resemblance in the creatures thus clothed, on 

 questions of wider import, forms the subject of the next 

 chapter, in which we shall also have to take into considera- 

 tion the conclusions reached in the previous one. 



