The Trend of Evolutionary Thought. 13 



shown to be capable of building an entire larva. And even the 

 frog's e^g, when suitably manipulated, yields an entire embryo 

 from one cell of the two-cell stage. Many other experiments 

 have been made by many workers, and the result is abundant 

 evidence in favour of the resourcefulness of the embryo and 

 against the idea that development is to be regarded as a mere 

 mechanical unfolding or " evolutio." As Russell says in his very 

 suggestive volume on " Form and Function," p. 346 : — " Experi- 

 mental work opened men's eyes to the fact that the developing 

 organism is very much a living, active, responsive thing, quite 

 capable of relinquishing at need the beaten track of normal 

 develo]>ment which its ancestors have followed for countless 

 generations, in order to meet emergencies with an immediate and 

 ])urposive reaction." 



Even modern mechanists have given up Weismann's view of 

 evolutio, and some of them lay stress, not on local determinants, 

 l)ut on ' the capacities of the reaction system." 



A fifth difficulty in regard to the mechanistic \\qw is seen in 

 the wonderful resourcefulness of the adult organism in circum- 

 stances that could not apparently have been provided for by 

 natural selection. Many experiments have brought home to us 

 the fact that organisms can deal successfully with new conditions. 

 The well-known power that the newt has of regenerating an 

 amputated limb might be accounted for by supposing there was a 

 considerable reserve of supplementary determinants placed at 

 convenient intervals along the member, but other cases are not 

 susceptible of such simple explanation. In the case of the 

 hydroid Tubularia, for instance, Miss Bickford has shown that 

 the combined work of many parts is involved in the restoration 

 of a decapitated head. And Driesch shows how the branchial 

 apparatus alone, or even half of the branchial apparatus of 

 Clavellina, may form a sphere, and then re-construct a small 

 complete ascidian. And again we have the classical case of 

 regeneration of the lens of an eye, not from the normal foun- 



