CONGRUITY OF THE EVIDENCE 303 



from those now living as to require to be arranged even in a 

 separate class from those which contain existing forms. It is only 

 when we come to the orders, which may be roughly estimated at 

 about one hundred and thirty, that we meet with fossil animals so 

 distinct from those now living as to require orders for themselves ; 

 and these do not amount, on the most liberal estimate, to more 

 than about lo per cent, of the whole." 



But the truth of the new views is still more strongly shown by 

 the light which they are capable of throwing upon another closely 

 related palaeontological problem of the greatest importance, which 

 has hitherto proved a stumbling-block to biologists and evolutionists 

 alike — that is, the fact of the existence of 'persistent types' of life 

 through long geologic ages up to the present day. As Huxley and 

 others have shown, this long persistence of similar organic forms 

 has been met with both among animals and among plants of com- 

 paratively high organisations. Certain genera of Mollusks, for 

 instance, are said to " have persisted from the Silurian epoch to 

 the present day with so little change that competent malacologists 

 are sometimes puzzled to distinguish the ancient from the modern 

 species." ' This persistency is assumed by some to be due to the 

 slow rate of change among such organisms, or to their having 

 passed into a rigid (as opposed to a plastic) condition ; and they 

 would have us believe that such organisms have been perpetuating 

 their kind, in the same likeness, through this long succession of 

 geologic ages. 



An excellent means of refuting this supposed explanation is 

 fortunately open to us, seeing that a similar persistency, through 

 many geologic ages, is known to obtain for two sets of very low 

 organisms closely akin to those which the writer has shown to take 

 origin so frequently by heterogenesis, and which are also notorious 

 for their high degree of variability. I refer to Foraminifera 

 (chambered Amcebae) and to Diatomaceae — very low types respec- 

 tively of the animal and of the vegetable kingdoms. It is the 

 sihceous envelopes of these low unicellular organisms, with their cha- 

 racteristic shapes and markings, that are preserved in the fossil 

 state, and which thus permit of an accurate comparison being made 

 of ancient and existing representatives of these simple forms of life. 



Concerning the former organisms Dr. Carpenter wrote,' " there 



is no evidence of any fundamental modification or advance in the 



' " Proceed, of Royal Institution," vol. iii., p. 151. 



" " Introduction to the Study of the Foraminifera," 1862, p. xi. 



