102 INTRODUCTION. 



it follows that the fossil forms belonging to any given group, in so 

 far as they are " generalised " in their characters, may fairly be said 

 to be " embryonic " types ; and as the oldest forms of any given 

 group are usually the least specialised, so they are likewise the most 

 " embryonic." It must be borne in mind, however, that if we speak 

 of fossil animals as being " embryonic types," we can only do so on 

 the distinct understanding that it is not thereby implied that they 

 were in any way degraded forms, or that they were at all less perfectly 

 constructed, or less thoroughly adapted for their surroundings, than 

 their modern representatives. 



(d.) Lastly, overwhelming evidence in favour of a general theory 

 of evolution is afforded by the similarity of the types of life in suc- 

 cessive faunae and florae. The animals and plants of each geological 

 system are more closely related to the animals and plants of the 

 system immediately below and to those of the system immediately 

 above, than they are to the organisms of any other rock-group in the 

 stratified series. This fundamental palaeontological fact does not 

 admit of reasonable explanation except upon the view that the 

 organisms of each geological period are the modified descendants of 

 those of the preceding period, and are the progenitors of the organ- 

 isms of the next succeeding period. Each geological system has, of 

 course, more or fewer special types of life, which are confined to it, 

 and which, apparently from inability to adapt themselves to changes 

 in their environment, die out before the close of the system without 

 leaving descendants. Others undergo but slight modification, and 

 appear in the next system as new species, " representative " of the 

 species from which they sprang. Others, again, vary more pro- 

 foundly, and break up into diverging groups, represented in the 

 succeeding period by more or less widely distinct forms. 



8. Gene?'al Progression of Organic Types. — The history of living 

 forms, as preserved in the palaeontological record, exhibits a distinct 

 upward progress from the lower to the higher, or from the more 

 generalised to the more specialised. At the present day, the animal 

 kingdom admits of division into a number of primary morphological 

 types, of which some are higher than others. Thus, the Vertebrate 

 type is zoologically higher than any type of the Invertebrates, and the 

 sub-kingdoms of the latter have also a certain relative rank according 

 to the complexity of their plan of organisation. In the same way, 

 within the limits of each sub-kingdom, some of the groups are more 

 " specialised," and therefore higher in the scale, than others. 



Not only do the primary morphological types differ from one an- 

 other in relative zoological rank, as estimated by relative complexity 

 of organic plan, but Palaeontology shows clearly that there has been 

 a progression in the order of their development, the lower types hav- 

 ing, in the main, preceded the higher in time. It is true, as before 



