306 EVOLUTION IN" BIOLOGY. 



animal life, commenced by Leeuwenhoek and Swammer- 

 dam, and continued by the remarkable labours of Reau- 

 mur, Trembley, Bonnet, and a host of other observers, 

 in the latter part of the seventeenth and the first half of 

 the eighteenth centuries, drew the attention of biologists 

 to the gradation in the complexity of organisation which 

 is presented by living beings, and culminated in the doc- 

 trine of the " echelle des etres," so powerfully and clearly 

 stated by Bonnet; and, before him, adumbrated by 

 Locke and by Leibnitz. In the then state of knowledge, 

 it appeared that all the species of animals and plants 

 could be arranged in one series ; in such a manner that, 

 by insensible gradations, the mineral passed into the 

 plant, the plant into the polype, the polype into the 

 worm, and so, through gradually higher forms of life, to 

 man, at the summit of the animated world. 



But, as knowledge advanced, this conception ceased 

 to be tenable in the crude form in which it was first put 

 forward. Taking into account existing animals and 

 plants alone, it became obvious that they fell into groups 

 which were more or less sharply separated from one 

 another ; and, moreover, that even the species of a genus 

 can hardly ever be arranged in linear series. Their 

 natural resemblances and differences are only to be ex- 

 pressed by disposing them as if they were branches 

 springing from a common hypothetical centre. 



Lamarck, while affirming the verbal proposition that 

 animals form a single series, was forced by his vast 

 acquaintance with the details of zoology to limit the 



