METAMORPHOSIS OF ORGANISMS. i 489 



we have of the control of the agent heat over development, than the well- 

 ascertained fact that the time of emersion of larvas depends upon the 

 temperature ? The silk-grower, by placing the eggs of the insect in an 

 ice-house, retards them as long as he pleases. The amputated limbs of 

 the water-newt can only be reproduced at a temperature from 58 to 75. 

 The tadpole, kept in the dark, does not pass on to development as a frog. 

 In decaying organic solutions, animalcules do not appear if light be ex- 

 cluded. 



Upon the whole, therefore, we conclude that organisms of every kind, 

 so far from presenting any resistance to change, are. im- Changes of or- 

 pressed without any difficulty by every exterior condition ; ^nd^n hiva" 

 and since existing natural circumstances have been main- riabie laws, 

 tained for a long time without any apparent change, their sameness pro- 

 duces a sameness in the order and manner of development. But it should 

 be borne in mind that this idea of, sameness can be entertained only on 

 an imperfect view of the state of Nature, for there is scarce one of those 

 conditions, to the sameness of which we have been referring, which has 

 not, in reality, undergone slow secular variations ; and with those changes 

 there have been changes in the manner of development. 



In truth, as I have on a former page observed, the only things which 

 are absolutely unchangeable are the laws of Nature, such, for instance, 

 as that of gravitation ; every thing else is to be looked upon as an effect, 

 or as a changeable phenomenon arising from the operation of those laws. 

 So, therefore, though, in this chapter, the terms physical in- Successive met- 

 fluences and natural conditions have been repeatedly used, cons^uence^f 

 yet a higher and more philosophical view of the case brings invariable law. 

 us inevitably at last to the idea of law ; and therefore I accept the in- 

 terpretation of all these facts, which has of late years been impressing 

 itself more and more strongly and clearly on the minds of physiologists, 

 that the development of every organism, from a primordial cell to its 

 final condition, however elevated that condition may be, is the inevitable 

 consequence of the operation of a universal, invariable, and eternal law. 



All animals, no matter what position they occupy in the scale of na- 

 ture, unquestionably arise in the first instance from a cell, which, possess- 

 ing the power of giving birth to other cells, a congeries at last arises, the 

 size and form of which is determined wholly by external circumstances. 

 In all cases, the material from which these cells are formed is obtained 

 from without, and, whatever the eventual shape of the structure may be, 

 the first cell is in all instances alike. There is no perceptible difference 

 between the primordial cell which is to produce the lowest plant and 

 that which is to evolve itself into the most elaborate animal. The mode 

 of growth, and the arrangement of the new cells as they come into exist- 

 ence, determining not only the form, but also the functions of the new 



