66 SECTIONAL ADDRESSES 



any time in the geological cycle, there is a marked increase in their mor- 

 tality coincident with the major physiographical paroxysms. Indeed, at 

 the Caledonian, Hercynian and Alpine ' revolutions,' something akin to 

 wholesale massacre overwhelmed once successful groups. Even when a 

 group, such as the Trilobites or Reptiles, survived such a storm, it did 

 so in greatly reduced numbers and importance. There are significant 

 exceptions to this common fate. The Ammonoidea, for instance, came 

 through the Hercynian revolution unscathed ; but they collapsed at the 

 first rumours of the Alpine troubles. Such exceptions are peculiarly 

 valuable in their relation to the phenomena of evolution, and will be 

 considered later. For the present we can be content to realise that the 

 bulk of evidence points to the fatal effect of environmental change on 

 a large proportion of the flora and fauna exposed to it. 



Environment has, then, a powerful influence for destruction ; but the 

 question as to its effect, if any, on the introduction of new types to replace 

 its victims is not so easily answered. The record of palseontological 

 succession certainly shows this replacement to be speedy and thorough. 

 The collapse of the Nautiloids in Hercynian times was compensated by the 

 rise of the Belemnoids, and the retirement of the Reptiles was followed 

 almost at once by the advance of the Mammals. The world seems never 

 to wait long for a full complement of novices to replace fallen veterans. 



One partial explanation of this is clear. Physiographical changes, by 

 depleting the ranks of the current population, reduce the incidence of the 

 biological factor. of the struggle for existence, so that active competition 

 is temporarily abated. Without competition, the offspring of the sur- 

 vivors have better individual chances of life, and multiplication with its 

 accompaniment of variation will be almost unrestrained. This explana- 

 tion, like most of its kind, leaves the main problem unanswered. It fails 

 to show why conditions that were fatal to one group should stimulate 

 another with similar habits and needs ; and it leaves open the question 

 as to the selection of one group for destruction and another for advance- 

 ment. Surely, if depletion of the population improves individual prospects 

 for the offspring of one race, it should have the same beneficent influence 

 on the next generation of any other with similar propensities, including 

 the race that has just been decimated. It would be absurd to postulate 

 that a group of organisms living and flourishing in all parts of the world 

 could have been immolated at one fell swoop by a universal cataclysm ; 

 so that there must be some other factor that decides between the doomed 

 and the preferred. For the moment we must defer further discussion 

 of this difficulty. 



The longevity of some types of organisms as compared with others 

 shows clearly that some are less- susceptible to the -lethal influence of 

 environmental change than others. We have already seen that the 

 types that weather the storms of time are those with relatively simple 

 structures, while those prone to collapse before them have more complex 

 structures. Both types of structure agree in their admirable suitability 

 in an appropriate environment ; but it is obvious that a wider range 



