VI EVOLUTION IN BIOLOGY 915 
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 expressed by disposing them as if they 
were branches springing from a common hypo- 
_ thetical 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 assertion to such a series as may be 
_ formed out of the abstractions constituted by the 
common characters of each group.! 
Cuvier on anatomical, and Von Baer on embryo- 
logical grounds, made the further step of proving 
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5 
' that, even in this limited sense, animals cannot be 
° 
_ arranged in a single series, but that there are 
‘several distinct plans of organisation to be observed 
among them, no one of which, in its highest and 
_ most complicated modification, leads to any of the 
_ others. 
1 Tl s’agit done de prouver que la série qui constitue 
Péchelle animale réside essentiellement dans la distribution des 
Masses principales quila composent et non dans celle des espéces 
_ Riméme toujours dans celle des genres.” —Philosophie Zoologique, 
chap. v. 
