58 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



COMPARISON OF GROWTH CURVES. 

 The Averages. 



Minot (1891) has shown, in the case of the guinea-pig, that growth 

 is rapid at first; and as the animal grows older a smaller daily incre- 

 ment is added. Stating it differently — as an animal grows older it 

 requires a constantly increasing span of time to add successive, equal 

 increments of weight, until finally growth ceases. This means that 

 the growth curve is steep at first, and that the early growth is the 

 greatest. Gradually the curve approaches a straight line, the adult 

 weight. The composite curves for both sexes (text-figures 1 and 2) 

 show this in the wild, the tame, and the hybrids. 



At the end of a year practically all of the animals were full-sized 

 adults; but in nearly all cases an extra 3 months was given to each 

 animal to follow a full compensation for any possible early retard. At 

 the end of a number of curves a slight unexpected increase appears. 

 This is due to the fat condition of a number of the older animals, as 

 the individual records show. One can easily follow any curve to its 

 logical conclusion. 



From an examination of tables 59 and 60 and their graphic repre- 

 sentation in text-figures 1 and 2, a number of salient facts, concerning 

 the average weights of the parent species and the hybrids, may be 

 recorded : 



(1) The average weights, and consequently the composite growth 

 curves of the wild, are well below the tame guinea-pig at all ages and 

 in both sexes. These do not show, however, that this is not completely 

 true for all individual weights of each species. For example, the indi- 

 vidual records reveal that some male guinea-pigs at the age of 10 days 

 were lighter than 95 grams. Although there was some overlapping 

 of the early individual weights of the wild and tame, as time progressed 

 the wild showed their specific character, and it required only a few 

 weeks before aU the wild were well below all the tame. Weights were 

 obtained for more than 4 wild males and 5 wild females. Originally, 

 composite curves were made including these. They only served to 

 augment the difference between the wild and the tame, for they were 

 sickly, did not thrive well in captivity, and died prematurely. Those 

 animals which entered into the tables and composite curves represented 

 in a fair way the natural growth of the wild Cavia rufescens. 



(2) The I wild hybrids of both sexes were remarkably vigorous 

 animals. The males attained an average which exceeded their larger 

 parent, the guinea-pig. They were also larger than all succeeding 

 hybrids. The females were likewise very vigorous. Curiously enough, 

 the middle portion of the composite curve of the females is below the 

 guinea-pig and the | wild. But, if an anticipation is permitted, it 

 will be shown that the bones of the | wild hybrids are larger than those 



