ANIMALS AND THEIR ENVIRONMENTS. 323 



of habits, as water-pools became less numerous. The young, 

 at first born alive and gilled, would be produced at less 

 frequent intervals and in fewer numbers, whilst they would 

 also be retained for longer periods within the parent-body. 

 This view accords with the actual detail of the animal's life. 

 For only two young are produced at a birth by these 

 animals ; the other eggs serving as food for the developing 

 minority. This latter remarkable feature of the sustenance 

 of the young by their immature brethren can, of course, be 

 regarded only in the light of an acquired condition, and as 

 one which has arisen out of the needs and necessities of the 

 species. 



The conclusions to which the earnest and unbiassed 

 student of Nature may arrive regarding any points involved 

 in his studies may very frequently be found to be greatly at 

 variance with the notions of natural law and order that 

 prevail in the world at large. But as Tyndall has well 

 remarked, "in the choice of probabilities the thoughtful 

 mind is forced to take a side;" and the attitude of the 

 seeking mind towards natural phenomena and their explana- 

 tion must ever be that of estimating causes by the likelihood 

 and value of the evidence brought to light. Judged by the 

 standard of once-popular faith, that the living things of the 

 world were created as we find them, the cases of the flat- 

 fishes and amphibians do not seem very promising, it must 

 be confessed. But the choice of a side admits of no 

 hesitancy here. The evidence that outward and mechanical 

 agencies, operating upon living bodies and correlating them- 

 selves with the forces and ways of life, are the causes of the 

 peculiarities we have noted, is too forcible to be for a 

 moment doubted. A peculiar form or shape of body, a rise 

 of land, and the influence of the "law of likeness" in 

 perpetuating the variations thereby produced, such are the 

 causes and means through which the greater portion of the 

 world of life has been and is still being moulded. How 

 much in any case may be due to the influence of outward 

 causes, and what amount of power we are to ascribe to the 



