650 INVERTEBRATA 



CHAP. 



Of course this conclusion was tacitly assumed by all evolutionary 

 zoologists thirty years ago, but few have paused to think what con- 

 sequences are implied in this conclusion. Among these few must be 

 reckoned Sedgwick (1909), who points out that on this theory it 

 follows that, as time goes on, the life-cycle must be ever tending to 

 grow more complex, since new phases are always being added to it 

 at its adult end. Another way of phrasing this conclusion is to say 

 that a new step in evolution usually takes place when the adults of a 

 species seek a new environment, and in reaction with it have their 

 structure modified. 



If we inquire whether the adults of a species do often seek a 

 new environment, then some very interesting evidence may be 

 adduced. Allen (1899), as a result of several years' painstaking 

 exploration of that portion of the English Channel lying in the 

 neighbourhood of Plymouth Sound, has shown that each marine 

 species has a particular type of bottom which is suited to it, and 

 which may be termed its home. On ground of this kind it swarms; 

 but around the areas of this type of bottom numerous stragglers of 

 the species are to be found. It seems clear that, from the home 

 population, crowds of colonists are for ever being sent forth which, 

 in most cases, i'ail to maintain themselves, but which may in rare 

 cases successfully establish themselves, and in this way a new race 

 or species may be produced. Every species, indeed, can be compared 

 to a tire in the midst of a dry prairie, which for ever tends to extend 

 its borders. 



IS'ow when an animal encounters a new environment one of two 

 tilings will result ; either its metabolism, and as a consequence all 

 its activities, will be checked ; or the metabolism will be promoted 

 and the vigour of its life increased. In the first case it will either 

 die or lead a stunted and sickly existence; in the second case its 

 structure will almost certainly be modified. This modification may 

 be described as a reaction to the stimulus of the new environment; 

 and as environment can be analysed into a few factors, such as food, 

 temperature, moisture, salinity, etc., the modification must, in the 

 last resort, be the effect of one or more of these on the metal lolism. 

 When, therefore, we find that the larva, as compared with the adult 

 ancestral stage which it represents, is almost always very much 

 reduced in size, and that the change to the adult condition takes 

 place whilst the animal is still very small, one receives the impression 

 that one is dealing with a reaction which, constantly repeated through 

 thousands, nay, myriads of generations, tends to set in sooner and 

 sooner in the course of the development ; just as in the life of the 

 individual, the formation of habit causes reactions to require for their 

 evocation less and less of the original stimulus. 



But, the reader will exclaim with horror, does not this explana- 

 tion postulate the acceptance of that Larnarckian heresy, the inherit- 

 ance of acquired characters ? and have not experiments shown such 

 an idea to be devoid of foundation ? The answer to this question 



