56 AN INTRODUCTION TO ZOOLOGY. 



far-away ancestor is dropped, and the character of the 

 immediate ancestor is attained. In other words, there 

 is a point in the development of an individual at which 

 it assumes the characters of its immediate ancestors ; 

 and if we "hark back" from this point, we come 

 across the characters of more remote ancestors. Wo 

 may reason from the analogy of these facts, and 

 draw similar conclusions regarding the origin of the 

 successive stages presented by the embryo in the egg. 

 For instance, when a hen's egg has been incubated 

 for three days, the neck of the enclosed chicken ex- 

 hibits on each side a set of holes or gaps, with straight 

 blood-vessels running between them. These are cor- 

 relative in their position, and in the distribution of 

 their blood-vessels, to the gills of a fish. At a 

 later stage these gill-clefts disappear, and the associ- 

 ated blood-vessels become altogether modified. ]f 

 the stripes on the black kitten are its inheritance 

 from the ancestral striped grey cat (which no one 

 will doubt), are not the gill-clefts of the three- 

 days-incubated chicken its inheritance from an an- 

 cestral fish, or fish-like animal ? Equally striking 

 is the case of those animals which undergo what is 

 termed metamorphosis ; which have, that is to say, 

 an early stage, called the larval stage, which is quite 

 different from the adult form in appearance and 

 habits. Familiar instances of this are the frocr, with 

 its larval stage tlie tadpole, and the butterfly, with its 

 larval stage the caterpillar. If we imagine the tad- 

 pole to remain closed within the egg until it was 



