It is shown that the experimental cross-section ratio of the reactions $\mathrm{pp}\to {\Delta }^{++}$ (neutrals) and $\mathrm{pp}\to {\Delta }^{++}\left({\pi }^{-}p\right)$ (at 6.6 GeV/c) for low momentum transfer to the ${\Delta }^{++}\mathrm{}\left(\right|t|<\mathrm{}0.2\mathrm{} \mathrm{Ge}{\mathrm{V}}^2)$ is approximately the same as the cross-section ratio $\frac{\sigma ({\pi }^{-}p\to \mathrm{neutrals})}{\sigma ({\pi }^{-}p\to {\pi }^{-}p)}$ for ${\pi }^{-}p$ effective mass less than 1.56 GeV. The disagreement observed between the ratios for masses greater than 1.56 GeV can be explained by increased values of $t_{min}$ and non-${\Delta }^{++}$ background in the $\mathrm{pp}$ reactions. These results imply that pion exchange dominates $\mathrm{pp}\to {\Delta }^{++}$ (neutrals) for low momentum transfer to the ${\Delta }^{++}$.

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