INTRODUCTION. 1 9 



those who try to diminish the number of genera do not act 

 consistently. It may be said that if two of the eggs of a single 

 Bryozoon may grow into different forms, those forms must be 

 regarded as the same species, and that to assign them to different 

 genera is subversive of all ideas of nomenclature. 



This raises the question whether there are such things as genera 

 and species among Cyclostomatous Bryozoa. 



If we take the Echinoidea, we find that the term genus has in 

 that class a fairly definite value. Two individuals belonging to 

 different genera may have had a common ancestor, but that 

 ancestor must have lived many thousands of generations ago. 

 For example, the two commonest living English Echinoids are 

 Echinus esculentus, L., and Echinus mitiaris, O.F.M. The dis- 

 tinctions between these species were nearly as well marked in the 

 period of the Crag (Lower Pliocene or Plaisancian) as they are at 

 present. They probably had as their common ancestor Echinus 

 serresi, Desml., from the Helvetian or Middle Miocene. Let x 

 represent the number of generations which lived in that division 

 of geological time known as an "age"; then these two 

 species have been distinct for at least 5# generations; and their 

 common ancestor lived Ix generations ago. Similarly with genera. 

 The closest ally of the genus Echinus is a group of small, 

 uniformly tuberculate species, to which palaeontologists give the 

 name Psammechinus : the differences between the two groups are 

 usually regarded as only subgeneric ; nevertheless, the two have 

 been distinct for 9# generations. 



This illustration reminds us that to find the common ancestor of 

 similar closely allied species of Echinoids, we have to go back a very 

 long way ; and to find the common ancestor of two subgenera, we 

 have to go back still further. But in the case of the Cyclostomatous 

 Bryozoa, great structural differences may be produced in only a 

 few, or perhaps even within a single generation. For example, in 

 the seas in which were deposited the Great Oolite of Normandy 

 and of the Cotteswold Hills, there lived many specimens of the 

 erect, frondose Bryozoa (here accepted as Diastopora] ; whereas the 

 encrusting specimens (Berenicea) were very rare or absent. The 

 conditions were favourable to erect forms, and all the young of 

 these frondose forms adopted the same mode of growth as their 

 parents. At Bradford, in Wiltshire, the geographical conditions 

 changed rather suddenly, and the Bradford Clay was deposited 



