VARIATION IN" PERSISTENT TYPES. 213 



their inability to cope with the less favourable conditions of ex- 

 istence to which, through one cause or another, they may be 

 subjected, others, under more favourable circumstances, and re- 

 quiring no adaptation to new conditions, will continue unchanged 

 as before. Hence, the same stock may continue in a direct line, 

 and yet, at the same time, throw off a number of side branches, 

 whose ulterior development may, or may not, keep pace with the 

 main stem. That the simple transformation of a group requires 

 either the immediate or the ultimate obliteration of that group, 

 has nothing to support it. Few naturalists at the jnesent time 

 question the descent of at least some of the races of the domestic 

 dog from one or more species of wolf or jackal, yet these are living 

 side by side, and apparently without interfering with one another's 

 wants. That a decadence in the one group or the other may ulti- 

 mately set in — in fact, has already set in — cannot be denied, but it 

 may be safely doubted whether, if it had not been for man's inter- 

 vention, extermination of either the wolf or jackal would have 

 preceded that of the dog. A struggle with competitors in a cer- 

 tain quarter of the globe, or the necessity of conforming to new 

 conditions of existence, may have developed specific characters in 

 some of the wolves which they had not hitherto possessed, and 

 which would not be necessary for more favoured forms occurring 

 elsewhere, or, possibly, living even in the same region. In the 

 same way that modification of the nautiloid type which, it is as- 

 sumed, has resulted in the formation of the genus Gonialites, does 

 not appear to have materially affected the parent stem, for we find 

 the genus Nautilus itself developing in almost equal abundance in 

 the periods succeeding the introduction of the goniatite as in those 

 preceding it. In the deposits of the Carboniferous formation, 

 which represent the period of maximum specific development in this 

 genus, we have, according to Zittel, eighty-four species represented; 

 in the Trias about seventy, in the Jura fifty, and in the Cretaceous 

 deposits still sixty to seventy. The actual downfall begins only 

 with the Tertiary, where through the entire series there are but 

 fifteen known species. What should have brought about this sud- 

 den decadence it is, of course, impossible to determine. Cases of 

 undoubted transformism and extreme persistence like the one here 

 instanced are undeniably rare, but this seeming rarity is to be at- 

 tributed in great part to our ignorance respecting the genetic rela- 



