VI.] SPECIES AND TIME. 157 



Thus, then, we find a wonderful (and, on Darwinian 

 principles, an all but inexplicable) absence of minutely 

 transitional forms. All the most marked groups, bats, 

 pterodactyls, chelonians, ichthyosauria, anoura, etc., appear 

 at once upon the scene. Even the horse, the animal whose 

 pedigree has been probably best preserved, affords no 

 conclusive evidence of specific origin by infinitesimal, 

 fortuitous variations ; while some forms, as the labyrintho- 

 donts and trilobites, which seemed to exhibit gradual 

 change, are shown by further investigation to do nothing 

 of the sort. As regards the time required for evolution 

 (whether estimated by the probably minimum period re- 

 quired for organic change, or for the deposition of strata 

 which accompanied that change), reasons have been sug- 

 geted why it is likely that the past history of the earth 

 does not supply us with enough: First, because of the 

 prodigious increase in the importance and number of 

 differences and modifications which we meet with as we 

 traverse successively greater and more primary zoological 

 groups ; and, secondly, because of the vast series of strata 

 necessarily deposited if the period since the Lower Silurian 

 marks but a small fraction of the period of organic evolution. 

 Finally, the absence or rarity of fossils in the oldest rocks 

 is a point at present inexplicable, and not to be forgotten 

 or neglected. 



Xow all these difficulties are avoided if we admit that 

 new forms of animal life of all degrees of complexity ap- 

 pear from time to time with comparative suddenness, be- 

 ing evolved according to laws in part depending on sur- 

 rounding conditions, in part internal similar to the way in 

 which crystals (and, perhaps from recent researches, the 

 lowest forms of life) build themselves up according to the 

 internal laws of their component substance, and in harmony 

 and correspondence with all environing influences and 

 conditions. 



