304 CONCLUSION 



Foraminiferous type from the paleozoic period to the present day." 

 Similar types and similar varieties from those types are, he said, to 

 be met with in geological formations existing as far back as the 

 triassic rocks.' And in reference to the same subject Pritchard ' 

 spoke as follows : " It may be generally stated that the relative 

 number of identical fossil and recent species is much greater in 

 this family of Foraminifera than in any other known ; and specific 

 forms have continued from the Mesozoic Era until the present day, 

 so connecting, as by an unbroken chain, the fauna of our own time 

 and that of almost countless ages past." Yet the variabiUty of 

 these organisms is extreme, as may be gathered when Dr. Car- 

 penter says : " The only natural classification of the vast aggregate 

 of diversified forms which this group contains, will be one which 

 ranges them according to their direction and degree of divergence 

 from a small number of principal family types." 



Turning now to the Diatomaceae, whose extreme variability is no 

 less notorious. Dr. Gregory 3 wrote as follows in reference to the 

 agreement of recent and fossil forms : " I have no hesitation in 

 saying that I believe all the forms in the ^Egina clay marl, which is 

 the oldest Diatomaceous deposit yet described, will be found living 

 on our coast. ■» ... It may also be observed that of all the forms 

 figured by Ehrenberg from more recent strata . . . the great 

 majority are perfectly identical with existing Diatoms." He sub- 

 sequently said : " Taking these facts into consideration, I am led to 

 believe that we have no evidence that any species of Diatom has 

 become extinct, as so many species, and even genera and tribes, of 

 more highly organised beings have done." Sir Joseph Hooker, 

 Pritchard, and other writers also bear witness to the cosmopolitan 

 distribution of these organisms. And the fact that they show 

 themselves in the most varied regions with identical anatomical 

 characters, seems to indicate pretty conclusively that the intrinsic 

 properties and varieties of the living matter of which they are 

 composed (that is, of their ' physiological units '), have more to do 



' But if certain low organisms developed into Foraminifera in remote geologic 

 ages, there is no reason why they should not develop in the present day into 

 essentially similar forms ; and variation may now tend to manifest itself in the 

 same fashion as it did formerly, owing to the fact that the causes (both intrinsic 

 and extrinsic) leading to this variation are essentially similar. 



' " History of Infusoria," 4th edn., p. 232. 



3 " Proceed. Roy. Soc. Edin.," 1856-7, p. 442. 



» The stratum at ^gina is said to belong either to the chalk formation, or to 

 the oldest tertiary or Eocene beds. 



