188 
convert those who deny a priori the possibility of solving 
a problem like that of the derivation of Vertebrates from 
Invertebrates How wide once seemed the gulf between 
Cryptogams and Phanerogams. Yet the bridge between them 
has been found. 
Ascidians. — The first step towards the solution of the 
problem of the origin of Vertebrates seemed to have been 
done with KOWALEWSKY's (1866) researches on the develop- 
ment of the Ascidians which revealed such remarkable 
points of agreement between these animals, formerly con- 
sidered as Invertebrates, and the Chordates. Pretty universally 
the view was gaining ground that in the Ascidians we have 
to look for the root of the Vertebrate genealogical tree. 
Yet, though in recent days it has found again an advocate 
in BROOKS (1893), this first step soon proved to take 
us no further than so many a mighty offensive in the 
present European war carried those who undertook them *). 
There still remained an unbridged gap between Ascidians 
and Invertebrates. 
Segmented Invertebrates. — While according to the above 
conception the metameric structure of the Vertebrates could 
have nothing to do with that of certain groups of Inverte- 
brates, others especially laid stress on this point of agree- 
ment between Vertebrates on the one side and Arthropods 
and Annelids on the other. It is a well known fact that 
already in the beginning of the nineteenth century GEOFFROY 
ST-HILAIRE (1822, p. 11) compared the Vertebrates to 
Arthropods walking upside down, as was necessary to assume 
in order to render possible the identification of the central 
nervous system in both. The same idea was carried on by 
LEYDIG (1864, p. 185) who compared the supra- and infra- 
oesophageal ganglia of Arthropods to a brain of Vertebrates 
pierced by the gut between the crura cerebri. 
Annelids. — In the same year, but independently, SEMPER 
and DOHRN (1875) then transferred the comparison from 
Arthropods to Annelids, no doubt an important step forwards. 
Thus was born the celebrated theory of the Annelidan af- 
finity of Vertebrates, which for a long time has occupied 
a dominating place in zoological work. DOHRN was led to 
it by general considerations, SEMPER by the discovery that 
E 
) This article, though published later, has been written during 
the great European war. 
