22 JOURNAL OF CONCHOLOGY, VOL. 12, NO. I, JANUARY, I907. 



Now it is obviously impossible to do more than postulate theoreti- 

 cally a common origin for all the individual members of a species 

 The publication of Darwin's "Origin of species" revolutionized our 

 ideas on the subject of phylogeny, and now that the evolutionary 

 theory is universally accepted by biologists we assume a common 

 origin, not merely for the members of a species but for those of 

 genera, families, etc. — perhaps for all organized beings. 



The "Standard" Dictionary gives the following: — "A classificatory 

 group of animals or plants subordinate to a genus, and having 

 members that differ among themselves only in minor details ot 

 proportion and colour, and are capable of fertile breeding in- 

 definitely." Here again we soon find ourselves in difficulties if we 

 attempt to take the definition seriously. Who is to decide what 

 are major and what are only minor differences ? Botanists are in 

 the widest disagreement as to the number of species of our British 

 brambles just because some consider as insignificant differences to 

 which others attach great importance. The " Standard " definition 

 moreover, by stating that species are capable of fertile breeding 

 indefinitely, suggests a resuscitation of the once prevalent idea that 

 the hybrids resulting from the crossing of two distinct species are 

 always sterile, whilst the production of fertile offspring proved that 

 the parents belonged to the same species. When we learn that 

 forms so distant as the ring-dove and the common pigeon have 

 been crossed and produced fertile offspring we shall see that we 

 must either give up any intelligible conception of species or dismiss 

 the claim that hybrids are sterile. This case is by no means unique. 

 There are numberless similar occurrences amongst both plants and 

 animals. The subject of the sterility of some hybrids is too large to 

 be discussed here, but it may be mentioned that it most probably 

 results from what we might term a slight degree of protoplasmic 

 incompatibility between the sexual elements that has been accident- 

 ally acquired during a prolonged period of disuse of interfertilization. 



There is another conception of species that will be more readily 

 understood if we take an imaginary case. Let us suppose that there 

 were spread out before us all the various forms of the shells of one 

 group — say Clausilia. These we imagine to comprise not merely 

 such as have been collected or described, but every form that at 

 present exists, including all the minor, scarcely recognizable modi- 

 fications. We now proceed to sort out this assemblage of shells 

 into groups, each group consisting of such forms as have uniform 

 characters, or consisting of different forms connected with one 

 another by a complete series of intermediate forms. Each group 

 would be sharply marked off from any other. At first we 

 should perhaps be prepared to assert without hesitation that each 



