216 EVOLUTION IN BIOLOGY VI 
The conclusions enunciated by Cuvier and Von 
Baer have been confirmed, in principle, by all 
subsequent research into the structure of animals 
and plants. But the effect of the adoption of 
these conclusions has been rather to substitute a 
new metaphor for that of Bonnet than to abolish 
the conception expressed by it. Instead of regard- 
ing living things as capable of arrangement in one 
series like the steps of a ladder, the results of 
modern investigation compel us to dispose them 
as if they were the twigs and branches of a tree. 
The ends of the twigs represent individuals, the 
smallest groups of twigs species, larger groups 
genera, and so on, until we arrive at the source of 
all these ramifications of the main branch, which 
is represented by a common plan of structure. At 
the present moment, it is impossible to draw up 
any definition, based on broad anatomical or — 
developmental characters, by which any one of 
Cuvier’s great groups shall be separated from all 
the rest. On the contrary, the lower members of 
each tend to converge towards the lower members 
of all the others. The same may be said of the 
vegetable world. The apparently clear distinction — 
between flowering and flowerless plants has been 
broken down by the series of gradations between 
the two exhibited by the Lycopodiacew, Rhizo-— 
carpee, and Gymnospermee. The groups of Fungi, - 
Lichenes, and Alge have completely run into one 
another, and, when the lowest forms of each are 
