272 SCIENCE PROGRESS 



called absolutely impossible. It does, however, show an ante- 

 cedent probability in favour of a much vaster vista of time. 



I think, however, if the data be examined more closely and 

 are duly correlated, they might throw some light on our basal 

 problem, and the methods by which our knowledge can be 

 advanced are but a continuation of those which Prof. Sollas 

 himself has so graphically described. Prof. Sollas is of opinion 

 that the fossil ammonites were not, as a rule, deposited where 

 we now find them by ocean currents, but that their occurrence 

 in any strata, in any considerable quantity, implies that they 

 actually lived in that region. One point, therefore, needs 

 emphasis. The difficulties with regard to the origin and 

 development of species are, by these discoveries, greatly magni- 

 fied. All over the world, in a small zone of the Jurassic, roughly 

 one thirty-third of the whole period, a species appears, lives, 

 disappears. How was it evolved, and what are the stages in its 

 evolution ? We must note the strong probability that the 

 species was evolved since the end of the period indicated by the 

 last zone, but how and where ? Where are the intermediate 

 stages by which it was developed from pre-existing types ? 



This aspect deserves special consideration. The sudden 

 appearance and disappearance of world-wide species is striking, 

 and gives rise to considerable speculation. The fact that such a 

 succession of commonly found species is continually found 

 without intermediate stages might, at first sight, tempt us to 

 deny the hypothesis of evolution and to say that intermediate 

 forms do not exist. Fortunately, however, it does sometimes 

 happen, particularly in the fossil forameniferae, which make up 

 the main substance of the chalk cliffs, that the change of organic 

 forms is so gradual that division into distinct species is difficult. 

 We must assume that the missing intermediate forms existed. 

 But where are they? Here is an ocean species, as Prof. Sollas 

 so pertinently remarks, like our contemporary spirula, the shell 

 of which is one of the commonest objects on the seashore. It is 

 found fairly plentifully in a particular zone of the Jurassic. Yet, 

 apparently, it arises from nowhere, and disappears suddenly. 

 Such a problem calls for investigation. The sudden disappearance 

 may, perhaps, be due to the advance of some predatory enemy. 

 But what about the appearance ? And would they suddenly 

 disappear all over the globe ? Assuming the facts to be as 

 stated, we have an admirable guide to help us to piece together 



