178 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
the type Chordata includes not only the true vertebrates, but also the 
Lancelet (Amphioxus), the tunicates, and Balanoglossus; this scheme 
is founded upon the embryological evidence. Among the inverte- 
brates even more remarkable examples have been observed. Such 
radically different types as the segmented worms and the shell- 
fish (Mollusca) are brought into relationship by their ontogeny and 
their closely similar types of larvae, as are also, though less distinctly, 
the brachiopods or lamp-shells, and the Bryozoa. The Horseshoe- 
crab, or King-crab, so abundant along our Atlantic coast, was long 
of uncertain affinities; originally referred to the Crustacea, largely 
because of its marine habits of life, embryology makes much more 
probable its relationship to the air-breathing scorpions and spiders, a 
result which has been examined previously from another point of view 
in connection with blood-tests. 
Even before the publication of Darwin’s Origin of Species one 
of the great stumbling blocks in the way of the theory of special crea- 
tion was the existence in a great many animals of rudimentary organs, 
or such as are so far reduced and atrophied as to be of no service to 
their possessors. An analogy employed by my lamented friend, 
Mr. Richard Lydekker, may be advantageously repeated here. Let us 
suppose that a screw-steamer, with longitudinal shaft leading aft from 
the engine-room to the stern, where it carries the propeller, should, on 
close examination, reveal many signs that it has originally been a 
“side-wheeler,” or paddle-boat. Recognizable remnants of paddle- 
boxes, of bearings for a transverse shaft, and the like, are found; what 
would be the inevitable conclusion? No one would maintain that a 
naval architect, in possession of his senses, in constructing a screw- 
steamer would deliberately introduce features which are useful and 
appropriate only in a paddle-boat., The only reasonable explanation 
would be that the vessel had originally been built as a paddle-boat and 
had subsequently been converted into a screw-steamer and in the 
conversion it had not been found necessary completely to eradicate all 
traces of the original construction. Obviously, the same reasoning 
_applies to rudimentary organs. The only satisfactory explanation of 
such useless remnants is that their possessors are descendants of 
ancestors in which those organs were fully functional. It seems quite 
absurd to assume that, in a separately and specially created animal, 
useless structures, reminiscent of other animals in which the same 
structures are useful and valuable, should be included, merely to 
indicate ideal relationships and community of plan. 
