565.3 397 
IV 
Fossil Apodidae 
VERY group of animals contains in itself its own record if we 
could but decipher it. The zoology of the future is bound 
under the fascination of this idea to devote itself to an ever closer 
comparative study for the express purpose of gaining insight into 
the lines of their past development. We have no royal roads, and 
the hopes which were held out to us a few years back that embryology 
would provide us with one have not been realised. It can only 
supply us with hints, the full meaning of which we must learn by 
other methods. Most hopeful of good result are those groups in 
which allied forms, recent and extinct, offer themselves in sufficient 
numbers for comparison. None can compare in this respect with 
the crustacea. There is an enormous wealth of known crustacean 
forms extending upwards from the very lowest fossiliferous strata 
and still swarming in every suitable part of the globe. The 
problem has long been to find a genealogical key to reduce this 
immense stream of organised life to some order of development. 
The first and most striking feature noted was the fact that all 
but the most extreme forms are segmented, and the natural inference 
was that the common ancestor of the whole immense family must 
have been some less specialised segmented form. Of recent years 
the attention of zoologists in search of the most primitive form 
among existing crustacea has been concentrated upon the freshwater 
Apus which appears sporadically in rain pools all over the world. 
When the pools dry up the eggs remain in the mud ; indeed it is said to 
be a necessary condition of development that they should be so dried. 
How long they remain capable of development is not known. These 
facts are interesting because they suggest to us a way in which Apus 
may have survived, practically isolated from the struggle for exist- 
ence, almost unchanged from the days when the crustacea first 
appeared on the planet. Those who claimed the primitive character 
of Apus were not disconcerted by the absence of fossil remains which 
could be definitely assigned to Apus. There was, of course, always 
the hope that such might be found, and further there was the strik- 
ing fact to which they could appeal that one of the most prolific of 
early crustacean forms, the Trilobites, possessed many characters in 
common with Apus. The great Paleozoic family of the Trilobites 
