1. Rees & Venable (2007; Journal of Ecology, 95, 926-936) correctly identified scaling relations across species between offspring size at independence and adult size as patterns needing theoretical explanation. They also correctly identified that Charnov's (1993; Life History Invariants, Oxford University Press) model did not provide an adequate explanation. 2. Rees and Venable attacked several opinions which they attributed to us, but which we do not hold, and which we did not express in the papers they cited. Here we clarify the main points where we agree and where we disagree with Rees and Venable. 3. Rees and Venable claimed that we interpreted cross-species correlations between traits as constraints on the evolution of life histories. This claim is wrong. We interpret correlation between traits as arising from coordinated evolution, just as Rees and Venable do. Our papers cited by Rees and Venable consciously avoided the terminology of constraints, and passages in them show clearly that we see coordination between traits as arising from natural selection. 4. A model we have been developing independently (Falster et al. 2008; American Naturalist, 172, 299-317) agrees with Rees and Venable in concluding that longer times to adulthood are not, in themselves, sufficient to predict a positive relationship between offspring size and adult size. Falster et al. (2008) shows that density dependence later during growth, together with size advantage persisting from offspring size, can in principle provide an explanation for a positive relationship, and for differences in the slope of this relationship between plants and mammals.