70 Royal Institution : — 



Ihing beings of the past differed from those of the present period ; 

 and again, that those of each great epoch have differed from those 

 which preceded and from those which followed them. That there 

 has been a succession of living forms in time, in fact, is admitted by 

 all ; but to the inquiry — What is the law of that succession 1 differ- 

 ent answers are given ; one school atfirming that the law is known, 

 the other that it is for the present undiscovered. 



According to the affirmative doctrine, commonly called the theory 

 of Progressive Development, the history of life, as a whole, in the 

 past, is analogous to the history of each individual life in the present ; 

 and as the law of progress of every living creature now, is from a less 

 perfect to a more perfect, from a less complex to a more complex 

 state — so the law of progress of living nature in the past, was of the 

 same nature ; and the earlier forms of life were less complex, more 

 embryonic, than the later. In the general mind this theory finds 

 ready acceptance, from its falling in with the popular notion, that 

 one of the lower animals, e. y. a fish, is a higher one, e. (j. a mammal, 

 arrested in development ; that it is, as it were, less trouble to make 

 a fish than a mammal : but the speaker pointed out the extreme 

 fallacy of this notion ; the real law of development being, that the 

 progress of a higher animal in development is not through the forms 

 of the lower, but through forms which are common to both lower 

 and higher : a fish, for instance, deviating as widely from the common 

 Vertebrate plan as a mammal. 



The Progression theory, however, after all, resolves itself very 

 nearly into a question of the structure of fish-tails. If, in fact, we 

 enumerate the oldest known undoubted animal remains, we find 

 them to be Graptolites, Lingulce, Phyllopoda, Trilobites, and Car- 

 tilaginous Jishes. 



The Girqjtolites, whether we regard them as Hydrozoa, Anthozoa, 

 or Polyzoa (and the recent discoveries of Mr. Logan would strongly 

 favour the o})inion that they belong to the last division), are cer- 

 tainly in no respect embryonic forms. Nor have any traces of 

 Sponyiada'. or Forcniiinifera (creatures unquestionably far below 

 them in organization) been yet found in the same or contempo- 

 raneous beds. Linyulce, again, are very aberrant Brachiopoda, in 

 nowise comparable to the embr3'onic forms of any mollusk ; Fhyl- 

 lopods are the highest Entomostracn ; and the Ilymenocaris vermi- 

 cauda discovered by Mr. Salter in the Lingula beds, is closely allied 

 to Nebalia, the highest Phyllo])od arid that which approaches most 

 nearly to the Vodnpthalmia. And just as Ilymenocaris stands be- 

 tween the other IJntomoslvdcn and the Podopthnhma, so the Trilo- 

 hita stand l)etween the IJntomostraca and the Edriopthalmia. Nor 

 can anything be less founded than the comparison of the Trilobiia 

 with embryonic forms of Crustacea ; the early development of the 

 ventral surface and its appendages being characteristic of the latter, 

 while it is j)recisely these parts which have not yet been discovered 

 in the Trilobita, the dorsal surface, last formed in order of develop- 

 ment, being extremely well developed. 



