  Species thermodynamics has been proposed in analogy to black hole
thermodynamics. The entropy scales like an area and is given by the mere
counting of the number of the species. In this work, we $\textit{derive}$ the
rules of species thermodynamics and explain how those $\textit{originate}$ from
standard thermodynamics. We consider configurations of species in thermal
equilibrium inside a box of size $L$ and show that the temperature $T$ of the
system, which plays a crucial role, is always upper bounded by the species
scale $\Lambda_{\rm sp}$. We highlight three relevant regimes: (i) when
$L^{-1}< T<\Lambda_{\rm sp}$, and gravitational collapse is avoided, the system
exhibits standard thermodynamics features, for example, with the entropy
scaling like the volume of the box; (ii) in the limit $L^{-1}\simeq
T\rightarrow \Lambda_{\rm sp}$ we recover the rules of species thermodynamics
with the entropy scaling like the area of the box; (iii) an intermediate regime
with $ L^{-1}\simeq T< \Lambda_{\rm sp}$ that avoids gravitational collapse and
fulfills the Covariant Entropy Bound; this interpolates between the previous
two regimes and its entropy is given merely in terms of the counting of the
species contributing to the thermodynamic ensemble. This study also allows us
to find a novel and independent bottom-up rationale for the Emergent String
Conjecture. Finally, we propose the $\textit{Black Hole - Tower
Correspondence}$, a generalization of the celebrated Black Hole - String
Correspondence. This provides us with a robust framework to interpret the
results of our thermodynamic investigation. Moreover, it allows us to
qualitatively account for the entropy of black holes in terms of the degrees of
freedom of the weakly coupled species in the tower.
