THE SPECIES 247 



in it we see the effect of the continually recurring tendency to 

 union which prevents the species from falling asunder. The 

 family provides that the perpetually renewed reciprocal action 

 of the treasure-store of genes continues vital and unified. 

 It is through the family that the fullest conceivable reciprocal 

 exchange of properties is ensured. 



If we develop further the picture of the species as I have 

 sketched it, we get a chain that stretches away back into the 

 past and forward into the future, beyond the eye's reach. 



To get an idea of the relations of the separate families 

 to one another, independent of the phenotype of the individual 

 organisms, we must set, one behind one another, copies of 

 the familiar picture of the bifurcating genealogical tree, and, 

 by means of common ancestors, run connecting lines from 

 one tree to the next. We get in this way a pillar-like, ascend- 

 ing, three-dimensional network, the meshes of which cross 

 one another in the most various ways. 



If we wish to add the distribution of the different genes, 

 we can imagine that the separate strands forming the meshes 

 are made of various coloured threads twisted together, which 

 continually separate from one another and come together 

 again. To each species we must apportion a definite number 

 of coloured threads, which give to the whole its characteristic 

 colour. In this way we get a picture of the stable genotype 

 of the species. 



All such pictures are merely aids to our restricted powers 

 of visualisation ; but they are very important, because they 

 admit of our imagining, on the lines of an actual model, people, 

 race and species as separate and yet belonging to one another. 



However this model be constructed, and however the 

 threads interweave, the family, which forms the knots of 

 the meshes, is throughout the true building-stone of the 

 whole. People, race and species are merely links between 

 families. 



