538 WILLIAM BATESON. 
have been raised and have been variously replied to, and in 
this condition the matter rests. By those who support it, it is 
assumed that the common feature of segmentation is so binding 
and unique a property as to suffice to link together groups 
whose morphology is otherwise widely different. 
In the following pages it is proposed to examine the pro- 
priety of employing the character of metameric segmentation 
as one of first importance in forming a phylogeny of this kind. 
And before referring to the evidence derived from the fact that 
the three characteristic features of Chordata are found in 
Tunicata and Enteropneusta, which are unsegmented forms, it 
will be best first to discuss the meaning of the phenomenon— 
“‘seomentation”’—for if resolved into its elements it will be 
found to be by no means a peculiar feature of a few groups, 
but rather the full expression of a tendency which is almost 
universally present. 
The term ‘“‘metameric segmentation” has been used to 
describe several anatomical features, which reach their highest 
development in the Annelids, the Arthropods, and the Verte- 
brata. If an attempt be made to reduce this expression to its 
simplest terms it appears to mean, in the first place, that cer- 
tain organs of the body are serially repeated from before back- 
wards, and in the second place that, in the case of the 
Vertebrates and Annelids at all events, the body cavity is at 
some period of life divided into a series of compartments, each 
of which is closed off from its neighbours. But when a more 
precise account of. this phenomenon is required, and when it 
becomes necessary to particularise as to which of the various 
organs of the body is thus repeated, difficulty at once arises 
from the fact that this repetition is irregular, and even within 
narrow limits may vary considerably. In the case of many of 
the errant Polychets all the mesoblastic organs, together with 
certain apparently serially homologous parts of the nervous and 
digestive systems may recur for a seemingly indefinite number 
of times in one individual, or even the whole animal may be 
repeated in a chain, thus giving the highest expression to 
the phenomenon. On the other hand, as in Lumbricus, 
