io THE GROWTH OF GROUPS 



one special character cannot be regarded as different from 

 a species, it is true that they are clearly marked by one 

 character only, but there are many other groups which 

 are marked off by the possession of two, three, or more 

 characters of their own. 



A TACIT ASSUMPTION IN THE TERM SPECIES 



Before inquiring how animals came to be in their 

 present state, it is well to know that state. We cannot 

 of course know this, for we cannot see the whole of 

 the species at a moment, we can only form an idea of it, 

 and this idea may be and often is erroneous. Every 

 time that a species is defined and the type specimen 

 laid down in a museum, the tacit assumption is made 

 that there are in the world a very large though unknown 

 number of animals like that type specimen. Since the 

 number is unknown we should not assume that it is very 

 large in every case. My opinion concerning type 

 specimens is as follows. The type specimens in our 

 museums are representatives of groups, but the groups 

 are of variable numerical size. Many of them no doubt 

 contain at the moment millions of millions of individuals, 

 others contain tens of millions, others only thousands or 

 hundreds. A considerable number of them have become 

 extinct since the time of their definition ten or twenty 

 years ago. By virtue of its specific name a small group 

 of animals which occupies merely the corner of a field is 

 made to appear equivalent to a vast multitude of another 

 kind which is spread over most of the earth. Similarly, 

 a group which made its first appearance a few years ago 

 and has since become extinct is made equivalent to a 



