S06 



THE CHANGING WORLD 



The unit of the taxonomist is the species, just as the unit of the 

 anatomist is the organ, and that of the physiologist a functioning 

 system of organs. Exactly what constitutes a species is still a matter 

 of controversy. Someone has said that a species is simply a com- 

 promise of opinions on the part of experts. A species represents a 

 real entity, nevertheless, for it is something that outlasts the mor- 

 tal individuals composing it. For our present purpose it may be 

 described as a group of individuals more like each other than they 

 are like any other individuals. 



According to a time-honored system, larger groups than species, of 

 increasing inclusiveness, are employed in classification, such as gen- 

 era, families, orders, classes, and 

 phyla. (See unit IV.) Linnaeus, 

 past master in taxonomy, regarded 

 species as entirely separate groups, 

 to be arranged as if in the pigeon- 

 holes of a desk. This was before the 

 evidences of evolution were as well 

 known as they are today. Connect- 

 ing links, however, have played 

 havoc with the pigeonhole idea in 

 classification. A good example of 

 connecting links, dating back to the 

 Jurassic Period, is Archacoptcryx, the 

 earliest known reptile-bird, sporting 

 feathers, teeth, and a lizardlike tail. 

 Biological literature is full of such 

 connecting links, plainly indicating 

 relationship and the occurrence of 

 evolution. 



The attempt to sort out different 

 species of such groups of organisms 

 as, for example, sedges, mosses, 

 grasshoppers, violets, or fishes, im- 

 mediately brings difficulty, because 

 the representatives of these groups 

 grade into each other. It takes a specialist to do it. The more 

 nearly two species are related, the fewer and finer are the diag- 

 nostic features that can be found and utilized to distinguish them. 

 The dipterologist, for example, is obliged to resort to very minute 



Archaeopteryx, the oldest known 

 bird, drawn from the Berhn specimen. 

 Note teeth, three fingers, feathers, 

 and a Hzardlike tail. The only other 

 known specimen is in the British 

 Museum. (After Parker and Has- 

 weU.) 



