xix HEREDITY 365 



did not exhibit, but which were possessed by a grand- 

 parent or remoter ancestor. Thus a lizard in growing a 

 new tail to replace one that has been lost has been known 

 to grow one with scales like those of an ancestral species. 

 To find out a lizard's pedigree, a wit suggests that we 

 need only pull off its tail. When such ancestral resem- 

 blance in ordinary generation is very marked, we call it 

 'atavism ' or "reversion," but of this there are many 

 degrees. A boy " takes after his grandfather " ; a horse 

 occasionally exhibits stripes, perhaps harking back to 

 those of a wild ancestor ; a blue pigeon like the ancestral 

 rock-dove sometimes results from crossing different 

 races of pigeons ; or a cultivated flower reverts to the 

 simpler and more normal wild type. But many of the 

 so-called ' reversions ' are due not to reawakenings of 

 characters which have lain latent for ages, but to a 

 coming-together-again of characters which have been 

 analysed apart in previous generations. This is well 

 illustrated among domesticated pigeons and rabbits, where 

 it is not difficult to repack the box which has been un- 

 packed in man's establishment of different races. 



Every animal is usually a little different from its 

 parents, and cannot be mistaken for one of its fellow- 

 offspring, except in cases of " identical twins " which are 

 believed to arise from one ovum divided into two separate 

 halves. The proverbial " two peas " may be very unlike. 

 Against the big fact of persistent hereditary resemblance 

 is the fact of variability. The relation between successive 

 generations is such that the offspring is on the whole 

 like its parents, but various causes producing change 

 diminish this likeness, so that we no longer say ' like 

 begets like," but "like tends to beget like." 



2. Theories of Heredity historical retrospect. Theories 

 of heredity, like those about many other facts, have been 

 formulated at different times in different kinds of intel- 

 lectual language theological, metaphysical, and scientific 

 and the words are often more at variance than the ideas. 



(a) Theological Theories. It was an old idea that the 

 germ of a new human life was possessed by a spirit, some- 

 times of second-hand origin, having previously belonged 



