40 THE AGE OE THE EARTH 



are recounilzed as the ancestors of the hltrher Malaco- 

 straca, and )et tliesc latter already existed — in small 

 numbers, it is true — side by side with the Phyllocarida 

 in the Devonian. The evolution of the one into the 

 other must have been much earlier. Here, as in the 

 Arthropoda, we have evidence of progressive evolution 

 amoni;- the hii^hest i^roups of the Class, as we see in 

 the comparatively late development of the Brach)ura 

 as compared with the Macrura. We find no trace of 

 the origin of the Class, or of the lari4;er i^roups into 

 which it is divided, or, indeed, of the older among the 

 small groupings into families and genera.^ 



Of the Arachnida, although some of the most won- 

 derful examples of persistent types are to be found in 

 this class, but little can be said. Merely to state the 

 bare fact that three kinds of scorpion are found in the 

 Silurian, two Pedipalpi, eight scor[)ions, and two spiders 

 in the Carboniferous, is sufficient to show that the period 

 computed by geologists must be immensely extended 

 to account for the development of this Class alone, 

 inasmuch as it existed in a highly specialized condition 

 almost at the beginning of the fossiliferous series; while, 

 as regards so extraordinarily complex an animal as a 

 scorpion, nothing apparent in the way of progressive 

 development has happened since. Professor Lankester 

 has, however, pointed out to me that the Silurian scorpion 

 Palaeophonus possessed heavier limbs than those of 

 existing species, and this is a point in favour of an 

 aquatic life like that of its near relation, Limulus. If 

 so, it is probable that it possessed external gills, not 

 yet introverted to form the lung-book. The IMerostomata 

 are of course a Palaeozoic group, and reach their highest 

 known development at their first ai)pearance in the 

 Silurian ; since then they have done nothing but dis- 

 api)ear graduall)-, leaving the single genus I^imulus, 

 immodihed since its first appearance in the Trias, to 

 represent them. It is impossible to find clearer evidence 



' For ail account of the evolution of llic Crustacea see the rresidcntial 

 Addresses to the Geological Society in 1895 and 1896 by Dr. Henry 

 Woodward. 



