No. 543] BIOTYPES AND PHY LO GENT 149 



change in the external conditions, so the process of de- 

 velopment of a species or of any of its component indi- 

 viduals may be arrested, altered or accelerated by similar 

 means. Tims variants arise, individual or racial, some- 

 times slight, sometimes marked, but necessarily within the 

 limits laid down by the specific properties of the biotypos 

 involved. This may be well illustrated by one of Jack- 

 son's discoveries about variation in the genito-ocular 

 ring of the common tropical sea-urchin, Tripiieustes. 

 In specimens from Florida and the West Indies, 36 per 

 cent, have only the two posterior oculars insert, 38 per 

 cent, have three (the left anterior plus the posterior) and 

 18 per cent, have four (right and left anterior plus the 

 posterior). Evidently then individual variants both ar- 

 rested and progressive are common but within very re- 

 stricted limits. Further than this it appears that in 

 Bermuda a racial variant can be distinguished, for in 

 specimens from that locality 61 per cent, have only two 

 oculars insert, 35 per cent, have three and only 2 per cent 

 have four. Now it matters not at all whether the Ber- 

 muda race is considered an arrested variant or the West 

 Indian form a progressive variant; the important fact 

 is the evidently marked but definitely limited racial diver- 

 sity. The study of such variants, however little light it 

 may throw on the immediate cause of their appearance, 

 is bound to help make clear the normal line of develop- 

 ment of the species to which they belong, and emphasizes 

 the definiteness and the significance of their diversity. 

 But if biotypes are really the fixed and unchanging ele- 

 ments which compose a species, the problem as to why 

 this diversity is so commonly definite and significant is 

 apparently simplified not a little by our knowledge of 

 their existence. 



It is unnecessary to suggest any other phylogenetic 

 Problems and the bearing of the study of genetics on 

 them, for if in these which I have suggested it has not 

 been shown that such study is helping us to understand 

 these problems better and is even indicating solutions, 



