72 THE THEORY OF EVOLUTION 



offers much the most satisfactory solution of the 

 problem that has yet been suggested. 



Embryology further shows that evolution is not 

 invariably an advance from lower and simpler to 

 higher and more complex types, but may be by way 

 of degeneration, and degradation. The adoption of 

 a parasitic mode of life is very apt to cause such 

 degradation, and some very remarkable instances of 

 the degeneration of parasites have been observed. 

 An instructive example that may be cited is that of 

 Sacculina, a nondescript creature that is parasitic 

 on certain species of crabs. The parasite is attached 

 to the body of its victim, underneath the tail, by 

 means of root-like fibres which penetrate and ramify 

 throughout the interior of the crab. The root-like 

 fibres absorb nutriment and convey it to the body 

 of the parasite, which is reduced to a mere bag, 

 without appendages, muscles, nervous system, sen- 

 sory apparatus, digestive tract, or any determinable 

 organs save those of reproduction. The creature 

 has the power of assimilating the nutritive juices 

 which are conveyed to it by the root-like filaments 

 from the body of its host, and the power of reproduc- 

 tion, and it must have some respiratory and excretory 

 capacity, though there are neither gills nor glands. 

 From an examination of the adult parasite alone, 

 it would be quite impossible to classify it and deter- 

 mine the type and class to which it should be re- 

 ferred, but embryology solves the problem. From 

 the egg is hatched a free-swimming larva, which 



