132 GENERAL EVOLUTION. 



ain-ranges, so have vast numbers of individuals occupying sucli 

 areas been destroyed. Important alterations of temperature, or 

 great changes in abundance or character of vegetable life over 

 given areas, would produce the same result. 



This part of the subject might be prolonged, were it necessary, 

 but it has been ably discussed by Darwin. The rationale of the 

 ^^ origin of species," as stated by him, may be examined a few 

 pages farther on. 



p. Of the Characters of Higher Groups. 



a. Relations of Structures. — The evidences of derivative origin 

 of the structures defining the groups called genera, and all those 

 of higher grade, are of a very different character from those dis- 

 cussed in relation to specific characters ; they are more diflScult of 

 observation and explanation. 



First : It would appear to be supposed by many that the crea- 

 tion of organic types was an irregular and capricious process, 

 variously pursued by its Author as regards time and place, and 

 without definite final aim ; and this notwithstanding the wonder- 

 ful evidences we possess, in the facts of astronomy, chemistry, 

 sound, etc., of his adhesion to harmonious and symmetrical 

 sequences in his modes and plans. 



Such regularity of plan is found to exist in the relations of the 

 great divisions of the animal and vegetable kingdoms as at present 

 existing on the earth. Thus, with animals we have a great class 

 of species which consists of nothing more than masses or cells of 

 protoplasmic matter, without distinct organs ; or the Protozoa. 

 We have then the Coelenterata (example, corals), where the organ- " 

 ism is composed of many cells arranged in distinct parts, but 

 where a single very simple system of organs, forming the only in- 

 ternal cavity of the body, does the work of the many systems of 

 the more complex animals. Next, the Echinodermata (such as 

 star-fish) present us with a body containing distinct systems of 

 organs inclosed in a visceral cavity, including a rudimental nerv- 

 ous system in the form of a ring. In the Molluscs to this con- 

 dition is added further complication, including extensions of 

 the nervous system from the ring as a starting-point, and a 

 special organ for a heart. In the Articulates (crabs, insects) we 

 have like complications, and a long distinct nervous axis on the 

 lower surface of the body. The last branch or division of ani- 

 mals is considered to be higher, because all the systems of life 



