president's address. 193 



But when we j?o back into past geological times we are confronted 

 with another problem, wliich at first sight seems as difficult to solve 

 as was the Gordian knot to untie, namely : were the great formations 

 (remains of which we find distributed over the whole globe) 

 once continuous and more or less contemporaneous, or are we to 

 consider them only as homotaxial ? The late Edward Forbes was the 

 first who suggested the latter view — in other words, that when we 

 find a bed of rock containing the same group of fossils in widely 

 separated geographical areas, we are not to consider that it is all 

 contemporaneous, but only representative or homotaxial, and that 

 long periods of time may really have separated them from one another. 



There is, at the present day, a growing conviction amongst Naturalists 

 that, as we know from the records of the earlier life-history of our 

 earth, there were far fewer classes and orders represented in the 

 older rocks, so also that the several geographical provinces had not 

 yet been evolved, and that there was only one marine province over 

 all the oceans of the globe. That with the gradual evolution of 

 varied climate, and changed surroundings, the marine, littoral, and 

 terrestrial faunas and floras became more and more differentiated, 

 until they reached the condition of specialization in which we see 

 them to-day. 



We can understand that if this uniformity of condition were con- 

 nected with a uniformity in temperature, extending over the greater 

 part of our earth, which may well have been the case in the earlier 

 periods of the past, the wideness of distribution which the faunas and 

 floras of the globe then enjoyed, when compared with the limited 

 areas occupied by existing ones, would be readily explained. 



The question naturally arises — Are we to consider that all repre- 

 sentative species occurring to-day in widely-separated areas have been 

 derived from a common ancestor ? I think, as a nale, we are bound 

 to do so, unless the representation be merely mimetic and not actual ; 

 and I believe this applies not only to differences of latitude and 

 longitude, but also to differences of time ; and that the further back 

 a genus dates in geological time, the wider proportionately will be 

 the extent of the geographical area occupied by its surviving 

 representatives. 



Turning now to the appearance in time of the two great 

 groups, the Lamellibranchiata and the Gastropoda, we find that 

 to the former, or, as they are now called, Pelecypoda (a much less 

 appropriate designation "i, more than a third of the known fossil 

 shells belong. They have been estimated at 6,000, and they probably 

 greatly exceed that number. The Asiphonate forms with an open 

 mantle appear to be the more ancient type, those with respiratory 

 siphons and closed mantle-lobes being more characteristic of the 

 Secondary and Tertiary periods. As far as their distribution in 

 time is concerned the former are certainly a most ancient group, 

 several genera being I'cpresented in Lower and Upper Cambrian 

 times — yet they attain their maximum to-day. 



Amongst the chief Paloeozoic forms may be mentioned Nucula, 

 Area, and Avicula ; the Mytilidie are also largely represented. The 



