THE SUCCESSION OF ANIMAL FORMS 187 



laws of early generalised types, and sudden and wide introduc- 

 tion of new forms, which we have seen in the case of the in- 

 vertebrates and the plants. 



Such facts as those to which I have referred, and many 

 others, which want of space prevents me from noticing, are in 

 one respect eminently unsatisfactory, for they show us how 

 difficult must be any attempts to explain the origin and succes- 

 sion of life. For this reason they are quietly put aside or 

 explained away in most of the current hypotheses on the sub- 

 ject. But we must, as men of science, face these difficulties, 

 and be content to search for facts and laws, even if they should 

 prove fatal to preconceived views. 



A group of new laws, indeed, here breaks upon us. (i) 

 The great vitality and rapid extension and variation of new 

 specific types. (2) The law of spontaneous decay and mor- 

 tality of species in time. (3) The law of periodicity and of 

 simultaneous appearance of many allied forms. (4) The 

 abrupt entrance and slow decay of groups of species. (5) The 

 extremely long duration of some species in time. (6) The 

 grand march of new forms landwards, and upwards in rank. 

 Such general truths deeply impress us at least with the conclu- 

 sion that we are tracing, not a fortuitous succession, but the 

 action of power working by law. 



I have thus far said nothing of the bearing of the prevalent 

 ideas of descent with modification on this wonderful pro- 

 cession of life. None of these, of course, can be expected to 

 take us back to the origin of living beings ; but they also fail 

 to explain why so vast numbers of highly organized species 

 struggle into existence simultaneously in one age and disappear 

 in another, why no continuous chain of succession in time can 

 be found gradually blending species into each other, and why, 

 in the natural succession of things, degradation under the 

 influence of external conditions and final extinction seem to be 

 laws of organic existence. It is useless here to appeal to the 



