1 78 READINGS IN EVOLUTION, GENETICS, AND EUGI:NICS 



the type Chordata includes not only the true vertebrates, but also the 

 Lancelet {Amphioxiis) , the tunicates, and Bala no gloss us; 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. 



