216 LEONARD DONCASTER. 
II. VARIATION AND DISTRIBUTION OF THE GROUP. 
The examination of the specific characters and the geographical distribution of the 
Chaetognatha leads to several points of interest. In the first place, it is found that most 
species are world-wide in their distribution, and are obtained in almost all the warmer seas; 
only a few species have been observed in very limited areas, and since Sagitta flaccida and 
S. regularis, which had hitherto been recorded only from the West Indies and Japan 
respectively, have now been found also in the Indian Ocean, it seems probable that some, 
if not all, the remaining local species will be known eventually to have a wider range. 
Individuals of the same species have as a rule the same characters in whatever part of 
the world they occur, but there are a number of exceptions to this rule; for example, 
the S. serratodentata from the Maldive Group had usually a greater number of teeth than 
those of the Mediterranean, and Aida records the same fact in respect of several species 
from Japan. But the characters, which are used to distinguish the species of the Chaeto- 
gnatha, are very variable in themselves, so that examples from the same locality differ 
considerably from one another, and it is sometimes a matter of difficulty to determine 
these species with certainty; for example, the individuals described above as S. tricuspidata, 
which have an arrangement of teeth different from that of the type, might be referred to 
S. hexaptera, in which the teeth were fewer than the normal, and so in other cases. 
In fact, it almost seems that the species in the Chaetognatha are not very definitely fixed, 
but graduate into one another to some extent, although they can be separated into several 
groups, which are very distinct; for example, Sagitta hexaptera, S. tricuspidata, S. magna 
and S. lyra form a well-marked group of large species, which can be separated at a glance 
from the type represented by S. hispida and S. regularis. 
The question of species in the Chaetognatha is an interesting one from the point of 
view of evolution, for in most seas a great number of individuals of various species are 
found together but all having, as far as we know, similar habits and living mingled 
together. Geographical isolation or differences of habitat apparently do not exist, and 
probably most species breed through the greater part of the year, so that there can be 
no separation by differences of breeding season. 
In many instances two species, living together, are so closely allied that it is very 
difficult to distinguish them, in which case it seems hardly possible that the separation 
can have been due to natural selection. Possibly the great variety of Chaetognatha found 
together, all living under the same conditions and with similar habits, may be best explained 
by supposing the species to be very ancient, and that the different species have arisen in 
different parts of the world and have become spread by currents or other means of dispersal, 
until they are found in all the seas where the temperature is sufficiently high. The 
characters by which the species are distinguished, such as hooks, teeth, proportions of the 
body and fins, ete., are very variable within certain limits, as has been shown above. If, 
then, a part of the ocean became partly or wholly separated from the rest by geological 
changes, in the course of time this variability would undoubtedly cause the fauna, so cut 
off, to become different from the remainder, and, when they again became intermingled, 
they would be classed as different species. The Chaetognatha offer this problem in a 
