Rieseberg (1997) suggested that hybrid fitness can be studied comparing the responses of hybrid and parental populations in selection experiments that incorporate the ideas of Stebbins (1959) and Lewontin & Birch (1966). Contained in that framework is the idea that hybrids by high heterozygosity are well prepared to meet new environments. Nonetheless, this is not seen in mesquites that mutate in the new niche. What is seen is that as heat load increases, leaves become smaller which is perhaps Lamarckian. The necessary and sufficient proof of backcrossing to spread the mutant is commonly called introgression in mesquites, but they are clinal races not species. The spread of superior private genes that swamp parents seems axiomatic.