220 



CHAPTERS ON EVOLUTION. 



XI. 



THE E VIDENCE FROM DE VELOPMENT (concluded). 



III. THE DEVELOPMENT OF MOLLUSCS, AMPHIBIANS, &c. 



THE attempt has been made in previous chapters to show that in 

 the development of living beings there lies an enormous store and 

 fund of evidence which goes either directly to support evolution 

 as a rational theory of the universe, or which, at any rate, aids us in 

 comprehending the causes which have, directly or indirectly, made the 

 world of life the wondrous thing it is. The result of our inquiries 

 has been to show that in the first beginnings of an animal's develop- 

 ment, and in its earliest phases of progress, there is an amazing likeness 

 to the early stages of every other animal's progress towards maturity. 

 But even after these early similarities have appeared, there may be 

 demonstrated in many groups a later likeness, which may often be 

 traced beneath forms of the most diverse kind. The progress of the 



living being is unquestionably, as 

 Von Baer aptly put it, one from 

 the general to the special. Thus a 

 sponge, a sea-squirt, and a man, 

 may and do agree in the essential 

 phases of their earliest develop- 

 ment. But the special features 

 of each group of sponges, sea- 

 squirts, and quadrupeds are soon 

 respectively assumed, and, finally, 

 there appear those more defined 

 structures which mark the comple- 

 tion of development, and which 

 land us within the class, order, 

 or even species to which each 

 belongs. Development may thus 

 be compared to a journey in which 

 all the travellers, or developing animals, start from a common point, 

 and in which all pursue at first a common path that shortly, however, 

 branches out into numerous diverging roads and routes, each leading to 

 the goal or destination of the race. Community of origin is proved by two 

 animals following the same beaten track for a longer or shorter distance; 

 dissimilarity arising when their pathways diverge and the route divides. 



FIG. 136. MUSSEL. 



Shell opened, showing ligaments, muscles (c c\ 

 and foot (/). 



