268 GEOLOGICAL BIOLOGY. 



this individual Magellania, that in the course of its individual 

 life it has developed from an embryo condition in which its 

 mature characters were not exhibited. By analogy we infer 

 that these other characters of the loop were evolved from an- 

 cestors in which they did not appear; but before asking how r 

 we observe that since the Silurian time the loop has ap- 

 peared on successive forms up to the present time, exhibiting 

 no greater differences than the ordinal or class characters in 

 the same line have exhibited. The first specimen which 

 exhibited a loop was distinct from previous forms by that 

 character, and this, wijh other characters, caused it to be 

 classed in a distinct suborder from all other forms. We use 

 the term evolution to express the idea of appearance of such 

 a character at first. All the various families of which we 

 speak, and all the various genera, whose history we mark by 

 range of life-period in the geological scale, had thus a place in 

 the scale when their first known representatives appeared ; 

 and whatever the characters may have been (in the present 

 case it is calcified brachial loops), every case is a case of first 

 known appearance of such character, that is, it did not ap- 

 pear before, and the evolution consists in its coming into 

 appearance on some organism whose supposed ancestors did 

 not exhibit the character. 



Evolution of Fundamental Characters Relatively Rapid The 

 facts, to be sure, may be considered as very imperfect, but 

 if we lengthen our lines backward we but lengthen the period 

 in which the character has been repeated by ordinary genera- 

 tion without modification sufficient to upset our classification. 

 Or if we extend the evolution over a hundred or a thousand 

 generations it merely reduces the amount of the increment for 

 each stage of the evolution. Thus we see that so far as 

 the evidence testifies, the evolution of those characters which 

 mark the differences between separate classes, orders, sub- 

 orders, and even some families of organisms, has taken place in 

 a relatively short period of time; taking as measure either the 

 rate of general progress in the differentiation of organisms, or 

 the length of the life-period of each particular genus or fam- 

 ily. This is in harmony with a law of evolution formulated 

 by Hyatt as given in a subsequent chapter (Chap, xviii.). 



